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IC 9066 



Bureau of Mines information Circular/1986 



Longwall Roof Support Technology 
in the Eighties 



A State-of-the-Art Report 



By Ernest A. Curth and Jeffrey M. Listak 




UNITED STATES DEPARTMENT OF THE INTERIOR 



-'4^ "^^^ ' ^*^ / '*'^ 




Information Circular 9066 



Longwall Roof Support Technology 
in the Eighties 



A State-of-the-Art Report 



By Ernest A. Curth and Jeffrey M. Listak 



UNITED STATES DEPARTMENT OF THE INTERIOR 
Donald Paul Model, Secretary 

BUREAU OF MINES 
Robert C. Norton, Director 







f1 



0( 



c\0 



b^ 



Library of Congress Cataloging in Publication Data: 



Curth, Ernest A 

Longwall roof support technology in the eighties. 

(Information circular ; 9066) 

Bibliography: p. 35-37. 

Supt. of Docs, no.: I 28.27: 9066. 

1. Ground control (Mining). 2. Longwall mining, l. Listak, Jef- 
frey M. II. Title. III. Series: Information circular (United States. 
Bureau of Mines) ; 9066. 



TN295.U4 [TN288] 622s [622'. 28] 85-600288 



CONTENTS 

Page 

Abstract 1 

Introduction 2 

Acknowledgments 2 

Estimates of roof support loads 3 

Longwall support load prediction 3 

Statutory standards 4 

Alternative analysis of roof support adequacy,.... 5 

Estimates of roof loads In terms of the geometry of various shield types 6 

Methods of Improving ground control on longwall faces 12 

Reduction of shield weight and simplification of functions 17 

Shield design and testing. 19 

Dust control and ventilation 20 

The role of the Bureau of Mines In development of longwall roof support 

technology 21 

Demonstration of shield-type longwall roof supports 22 

Demonstration of longwall mining 23 

Longwall mining In steeply dipping seams 26 

Longwall mining of thick seams 28 

Single-pass mining 28 

Multlllf t working 29 

Sublevel caving 31 

Thin-seam longwall mining 31 

Conclusions and summary 32 

References 33 

Appendix A. — National Coal Board Mining Department Instruction PI/1982/6: 

The use of powered supports on longwall faces 38 

Appendix B. — Specifications for Thyssen RHS 12/30 shield 40 

Appendix C. — Manufacturers 42 

ILLUSTRATIONS 

1. Concept of roof block and forces 3 

2 . Shield diagrams 6 

3 . Lemnlscate tracks 7 

4. Lemnlscate shield 7 

5 . Double-telescoping leg 8 

6. Four-leg shield 10 

7. Roof shield 10 

8. Chock shield 11 

9. V-type shield U 

10. Load distribution on a canopy 13 

11. Upswept sliding canopy extension 13 

12. Piano key selector 15 

13. Bi-dl rotary valve, 15 

14. Remote batch control 16 

15. Floor pressure through different bases 16 

16. Effect of extension ratio on yield load 18 

17. Equalization of support force for shields with inclined legs 18 

18. Equalization of tensional force in link bars by substituting hydraulic 

cylinders 19 

19. Schematic of testing bending and torsional loads in the structure 20 

20. Side shield 21 



ii 



ILLUSTRATIONS — Continued 



Page 



21. 
22. 
23. 
24. 
25. 
26. 
27. 
28. 
29. 
30. 
31. 
32. 
33. 
34. 
35. 
36. 



1. 
2. 
3. 



Effect of dust seals , 

Caliper shield 320 HSL , 

Leraniscate shield 18/30 , 

Lemniscate shield 12/30 , 

Snowmass shield , 

Aligning shield in pitching strata , 

Tensional in-f ace anchorage , 

Two-leg high-seam shield , 

Two-bench mining method , 

Multilif t system 

Chock shield , 

Mid-Continent shield 

X-type shield 

Six-leg shield 

Plow face schematic , 

Plow face 

TABLES 

Longwall-ground-control-related accident experience 1977-83 

Evolution of roof cover and load density by face support systems. 
Comparison between early and modern shields 



21 
22 
24 
25 
27 
27 
28 
29 
30 
30 
30 
31 
33 
33 
34 
34 



2 
12 
17 





UNIT OF MEASURE ABBREVIATIONS USED 


IN THIS REPORT 


cm 


centimeter 


m3 


cubic meter 


Gmt 


billion metric tons (gigaton) 


mm 


millimeter 


kg 


kilogram 


MPa 


megapascal 


kN 


kilonewton 


m/s 


meter per second 


kN/m2 


kilonewton per square meter 
(kilopascal) 


mt 


metric ton 






mt/m^ 


metric ton per square meter 


kPa 


kilopascal 










mt/m^ 


metric ton per cubic meter 


L 


liter 










N/cm2 


newton per square centimeter 


L/min 


liter per minute 










pet 


percent 


m 


meter 










s 


second 


m2 


square meter 







LONGWALL ROOF SUPPORT TECHNOLOGY IN THE EIGHTIES 

A State-of-the-Art Report 
By Ernest A. Curth ^ and Jeffrey M. Listak ' 



ABSTRACT 

It took only 9 years from the first appearance of roof shields on the 
U.S. longwall mining scene to the present predominance of shield faces. 
An apparent consequence is the welcome downward trend in accidents re- 
lated to failures in longwall ground control. The report addresses load 
prediction, the effects of shield geometry on support loads, factors 
contributing to ground control, and related techniques. The Bureau of 
Mines took an active part in the evolution of longwall mining, including 
developing one of the first shield faces in 1975, the first lemniscate- 
type shields in 1976, the Mine Roof Simulator completed in 1980, the 
first shields in steeply pitching coalbeds in 1981, and the first multi- 
lift working of a thick coalbed in 1982. The information offered in 
this state-of-the-art report will assist in establishing criteria for 
roof support selection. 



Mining engineer, Pittsburgh Research Center, Bureau of Mines, Pittsburgh, PA. 



INTRODUCTION 



In terms of roof support technology , 
the introduction of shields in the seven- 
ties and eighties was a major step for- 
ward. Table 1 indicates the predominance 
of shield faces on the U.S. longwall min- 
ing scene and its apparent consequence, a 
welcome downward trend of ground-control- 
related accidents. 

The rapid progress of longwall technol- 
ogy began in 1975 and is highlighted by 
a development leading from the caliper 
shield to the modern lemniscate type. 
Shields have been gaining preference over 
chocks since their advent to longwall 
mining in 1975. Safety and productivity 

factors favoring shields over chocks 
are — 

1. A sheltered working space requiring 
minimum cleanup work. 

2. Structural stability that allows 
advancing without delay even with brush- 
ing roof contact and, by controlling all 
lateral loads , removing the requirement 
for cumbersome leg restraint and restora- 
tion devices. 

3. Wide hydraulic range of mining 
height. 

The development of roof shields in the 
Federal Republic of Germany preceded and 
paralleled their rapid adoption by U.S. 
miners. In 1983, 87 pet of the total 
production in Germany came from shield 
faces (1).^ In Great Britain, the Na- 
tional Coal Board introduced the first 
shield support system in 1977 to initiate 
Advanced Mining Technology (ATM) and 
Heavy Duty Mechanization (HDM) programs; 
as of mid-1982, 44 shield faces were 
operating, or 7.5 pet of a total of 581 
faces (7). 



TABLE 1 . - Longwall-ground-control- 
related accident experience, 
1977-83 



Census of 






Shield 


Ground- 


longwall 


Total 


Shield 


faces. 


control- 


faces 


faces 


faces 


pet of 
total 


related 
accidents 1 


1977 (2). 


77 


15 


19 


143 


1978 


(2) 


(2) 


(2) 


59 


1979 (3). 


91 


40 


43 


60 


1980 (4). 


105 


57 


54 


89 


1981 


(2) 


(2) 


(2) 


73 


1982 (5). 


112 


93 


83 


87 


1983 (6). 


118 


99 


84 


71 



^Source: Health and Safety Analysis 
Center, Mine Safety and Health Admini- 
stration, U.S. Department of Labor. 

2Census not available in 1978 and 1981. 

The Bureau of Mines has taken a leading 
part in the introduction of shields to 
the U.S. mining industry through technol- 
ogy transfer seminars, studies, and par- 
ticipation in several longwall demonstra- 
tions including one of the first shield 
faces in 1975 (8) and the first shields 
with lemniscate gear in 1976 ( 9_) . 

The first part of this report presents 
load predictions, estimates of loads in 
terms of the geometry of various types 
of shields, and factors contributing to 
ground control and related technology. 
The second part highlights the scope and 
variety of the Bureau's role in the prog- 
ress of longwall mining technology. The 
objective of this state-of-the-art report 
is to assist mine operators in select- 
ing a roof support system fitting site- 
specific strata conditions. 



ACKNOWLEDGMENTS 



The authors are indebted to the rep- 
resentatives of Dowty Corp, , Warren- 
dale, PA; Heintzmann Corp,, Lebanon, VA; 
Hemscheidt America, Pittsburgh, PA; Min- 
ing Progress, Ine, , Charleston, WV; and 

^Underlined numbers in parentheses re- 
fer to items in the list of references 
preceding the appendixes . 



Thyssen Mining Equipment Division, Mar- 
ion, IL, for their cooperation in the 
preparation of this report and gratefully 
acknowledge the permission granted by 
Gluckauf , Essen, Federal Republic of Ger- 
many, to use figures 17, 18, 19, 26, and 
27; and by Bergbauforschung GmbH, Essen, 
Federal Republic of Germany, to use fig- 
ures 3, 8, 15, 16, and 21. 



ESTIMATES OF ROOF SUPPORT LOADS 



LONGWALL SUPPORT LOAD PREDICTION 

A premlning Investigation includes core 
drilling to provide data for isopachs of 
various strata intervals and the over- 
burden thickness. Physical properties 
of rock are determined either from the 
cores (10) or directly by geophysical 
logging to obtain indicators of rock mass 
behavior. 

Where the underground site is acces- 
sible, roof and floor bearing-capacity 
tests can be carried out to determine 
contact area requirements for roof sup- 
ports and cutting pattern (9). For exam- 
ple, a soft underclay that fails at 210 
N/cm^ must be covered by leaving a layer 
of coal thick enough to prevent sink- 
ing of the supports. Conversely, soft, 
friable roof material may be kept from 
spalling by leaving a layer of roof coal. 

Several methods have been conceived to 
arrive at an estimate of the mean load 
density required to support the roof of a 
projected longwall site: 

1. Barry ( 11 ) proposed that the needed 
mean load density at yield can be esti- 
mated by considering strata separation 
and cantilever action in a stiff strata. 

2. Wilson ( 12 ) conceived the immedi- 
ate roof above a longwall face as a free 
block that must be supported (fig. 1). 



W^jJUVL Jll=K\'~\A>J>^K^ ' ~=J. 




TTxrrs'r^m 



The face break is assumed 
caving angle along a line 
takes place. Rock quality, 
jointing of the roof strat 
angle. A caving angle of 
tive of a very friable roof 
dicates friable strata, 
medium firm, and 45° and 
firm and very firm rock. 
The load density (R) in me 
square meter is calculated 



to occur at a 

where caving 

bedding, and 

a affect this 

0° is indica- 

rock; 15° in- 

30° indicates 

60° indicate 

respectively, 
trie tons per 
by (13) 



W 



R = TT- (L + H tan a), 
zr 



where W=LxHxcxd weight of free 
face block, mt , 

c = roof support centers, m, 

d = density of roof material, mt/ 
m^ , 

L = span from face to canopy rear 
end, m, 

r = length from face to the loca- 
tion of the resultant roof 
support force, m. 



and 



H = caving height, m, 

a = caving angle, degrees. 



Wilson assumed that the caving rock occu- 
pies 1.5 times the volume of the virgin 
strata and thus the caving height to 
the bridging strata is H = 2h, twice the 
extraction height h. The bridging beds 
form a beam supported on the coal face 
and the compacted gob, Wilson also in- 
troduced a corrective equation for in- 
clined seams: 



R = 



-( 



sin6 
tan6 



+ cos6 



j mt/m2. 



where 



and 



FIGURE 1. - Concept of roof block and forces. 



6 = inclination of coalbed, 
degrees 

tan6 = 0,4 friction coefficient 
between rock beds. 



3. Wade (L4) 
of more friable 
1.25 times the 
thus accounting 
traction height, 
lowing formula 
(MLD): 



assumed an immediate roof 
nature that occupies only 
volume of virgin strata, 

for four times the ex- 
He developed the fol- 

for mean load density 



MLD = (1 + I Cp) X 4dh mt/m^, 
n=l 

where h = mining height, m, 

d = density of roof rock, mt/m^ , 



and 



Cp = magnification coefficients. 



Magnification coefficients are identified 
as hanging of immediate roof, local face 
activity, bridging of immediate roof pri- 
or to first fall, main roof weight, and 
extended downtime. 

Determination of load density, proposed 
by Barry, has found application for most 
U.S. longwall designs, including Bureau 
of Mines-sponsored demonstrations such as 
the shields in the Kaiser Steel Mines and 
the Old Ben project in Illinois. Wil- 
son's method is mostly used in the United 
Kingdom and formed the basis for recently 
issued instructions of statutory effect 
(appendix A). Wade's concept is the most 
conservative one; it involves significant 
factors of magnification and therefore 
leads to high estimates. 

STATUTORY STANDARDS 

Guidelines for the approval of caving 
longwall operations were issued in the 
Federal Republic of Germany in 1966. The 
required minimum load density is consid- 
ered a function of extraction height in 
coalbeds under 18° of pitch. A correc- 
tive equation is introduced for pitches 
exceeding 18°. The required load den- 
sity (A) is calculated by the following 
formula (15) : 

A=1.6x2x2.5M=8M mt/m2, 
where M = coalbed thickness, m, 

2.5 = density of strata, mt/m^ , 



1.6 = a safety factor, 

and 2 = a factor allowing for cav- 
ing of roof strata to a 
height twice the thickness 
of the coalbed. 

For pitching coalbeds the formula becomes 
(15) 

A = (5 + 0.15 E) M, 

where E = pitch, grad (1 grad = 0.9°) 

The above formulas were conceived for 
the frame and chock supports of the six- 
ties. With the advent of shields in the 
seventies, manufacturers and mine opera- 
tors chose to raise the required mini- 
mum load density to 15 M mt/m^ from the 
statutory 8 M mt/m^ to account for the 
inclination of shield legs resulting in a 
mechanical disadvantage (15). 

Mandatory directives issued by the 
Federal Republic of Germany ( 16 ) in 1977 
address — 

1. Dimensions of the travelway, which 
shall not be less than 0.6 m in width 
and 70 pet of the extracted thickness in 
height. 

2. Control of a roof support from its 
neighbor, 

3. Deadman controls, 

4. Dust control by water sprays and 
dust seals between units. 

5. Powered face sprags for extraction 
in excess of 2.4 m. 

6. Examination and approval of each 
prototype at the material testing center 
maintained by Nordrhein-Westf alen State, 

In the United Kingdom the National Coal 
Board issued Guideline Pl/1982/6 titled 
"The Use of Powered Supports on Longwall 
Faces" in 1982 (appendix A), The new 
statute supersedes earlier (1966) safety 
standards that called for a maximum prop- 
free front of 2 m and includes — 

1. Setting and yield load densities in 
different longwall face zones. 

2. Maximum support centers. 

3. Efficient hydraulic system. 

4. Span of canopy tip to face in dif- 
ferent extraction heights. 



5. Powered forepoles in coal higher 
than 2.5 m and for a web deeper than 
0.8 m. 

6. Powered face sprags in coal higher 
than 2.3 m. 

The instructions list statutory minimum 
load densities in different face zones. 
The statute reflects Wilson's thoughts 
on determining load densities (17) . To 
maintain the immediate strata intact, the 
support must carry the weight of a free 
block of immediate strata extended to a 
height of 2H, where H is the extracted 
coalbed thickness. Thus, load density 
for setting (Ag) — 



As = 



2 H X 2.5 = 5 H mt/m2 



where 2.5 = strata density, mt/m^ 



and 



H = extraction height, m. 



As supports are lowered and advanced, 
each adjacent support has to bear an ex- 
tra load of 1.5 times the original load: 

1.5 X 5 H= 7.5 H mt/m2 

The yield valve is designed to maintain 
maximum support resistance without damage 
to the structure. The worst condition, 
occurring after the shearer pass and be- 
fore the support is advanced, may last 
for a considerable time, so a safety fac- 
tor of 2 is applied. Thus, load density 
at yield (Ay) is — 

Ay = 5 H X 1.5 X 2 = 15 H mt/m2. 

Statutory minimum load densities are 
listed for different face zones that des- 
ignate face portions of advancing long- 
walls, most common in European coal min- 
ing. The face line zone is flanked by 
two buttress zones that form the bound- 
aries against the pack holes and often 
feature roof supports of greater strength 
than those along the face line. Pack 
zone supports secure the pack building 
activities. They frequently are equipped 
with rear canopy extensions and extra 
legs. The roadhead zone supports ex- 
tend over the gateroads, where face-end- 
forming activities are concentrated in 
a congested space. Since packwall and 



roadhead supports are located at the face 
ends , it is thought that roof behavior 
requires face end supports to be of only 
two-thirds the yield load density of the 
face line zone supports, or 10 H mt/m^. 

In the United States, Mandatory Safety 
Standard CFR 75.201-3 calls for approval 
of the roof support system of the long- 
wall on an individual basis. A roof con- 
trol plan must be submitted to the 
District Manager of the Mine Safety and 
Health Administration. The plan usually 
includes number, type, and capacity of 
the roof supports, and the method of the 
recovery at the termination of a panel. 

ALTERNATIVE ANALYSIS 
OF ROOF SUPPORT ADEQUACY 

British and German statutory standards 
determining load densities as a function 
of extracted seam height are, of course, 
arbitrary. A load density of 20 times 
seam thickness for a mining height of 
1 m, or 20 mt/m^, may not be adequate at 
all, but a value of 10 times thickness in 
a 3-m coalbed, or 30 mt/m^, may be fully 
satisfactory (18) . Rather than mining 
height, the German research center in Es- 
sen targets the area occupied by cavities 
greater than 0.3 m in height in an ob- 
served roof area to provide estimates of 
roof control adequacy. The information 
is derived from data collected on numer- 
ous faces through the Essen center's 
longwall face surveyance method and sta- 
tistical evaluation (19) . 

These data are processed by an analysis 
of variance with a view of obtaining a 
criterion for roof control through the 
interaction of following parameters: 

1. Thickness of shale overlying the 
coalbed, less or greater than 2 m. 

2. Rock pressure, MPa, as calculated 
(20). 

3. Mean load density of roof support, 
kPa. 

A. Mean span canopy to face, m. 
The result is an estimate of the percent- 
age of the area of cavities higher than 
0.3 m in a mapped roof area. Roof con- 
trol is more than adequate if such cavi- 
ties occupy less than 10 pet of the roof 
area under observation. 



ESTIMATES OF ROOF LOADS IN TERMS OF THE GEOMETRY OF VARIOUS SHIELD TYPES 



Figure 2A is a schematic of a two- 
leg caliper shield of the type installed 
in New Mexico in 1975 (8^). The canopy 
tip describes a circular arc when the 
shield is raised or lowered. Hence, the 
critical span between canopy tip and 
face widens with increasing height 
of the shield unless compensated by 
an extension. When yielding, caliper 
shields develop a horizontal thrust to- 
ward the face and an increase in vertical 
load because the friction between canopy 
and roof rock must be overcome. The 
friction coefficient between steel and 
rock is estimated to be 0.3 = tan 16.7°. 

The two legs are arranged between 
gob shield and base so that the support 
force is introduced into the hinge be- 
tween canopy and gob shield. The me- 
chanical disadvantage due to the shield 
geometry is a function of the extraction 
height and reduces the force acting on 
the canopy hinge. The efficiency of this 
caliper system is quite low, and the 
force available at the canopy is re- 
duced to 50 pet of the leg force in some 
cases. 

Support force and mean load density can 
be estimated by following formulas: 



A = 



_ 2 T e cosy 



A = 



2 S e cosy 



setting, cosy = 1 



^(/i=0) 




A, Caliper 9, Lemniscate 

FIGURE 2. - Shield diagrams. 



A = 



A.U ^ 






Aui ^ 



_ 2 T e cosy 



where 



and 



2 S e cosy 

fg C M 

2 T e cosy 
f 3 C M 

2 T e cosy 
fn C M 



at yield point, 
cosy = 1 

when yielding, 
cosy = 0.96 

setting, cosy = 1 



at yield point, 
cosy = 1 

when yielding, 
cosy = 0.96 



S = setting load per one leg, 
kN, 

T = yield load per one leg, kN, 

0.3 = friction coefficient be- 
tween steel and rock, 

y = tan "10.3 = 16.7°, 

A = vertical component of roof 
support force in the can- 
opy hinge, kN, 

C = length of canopy plus span 
of canopy tip to face, m, 

M = shield spacing, m. 

Ay, = mean load density, kN/m^, 

e, fg, fn ^re measured for each 
working height, m. 



Figure IB is a schematic of the early 
lemniscate shield installed in Illinois 
in 1976 (Jl.) • The canopy tip of a lem- 
nlscate-type shield moves up and down in 
a nearly straight line that is the cen- 
ter part of a lemniscate or figure-eight 
curve; therefore, the span between canopy 
tip and face barely changes. Figure 3 
shows the relationship between the lem- 
niscate curve described by the canopy 
hinge point, the moving tracks of the 
pivotal link bars, and the pole where the 



t 




• 


0) 


1 


i^ 


o> 


N 


'Ap'-.--?- 


c 




"*^ 


o 




5 


(T 




/ 


1 




/ 

/ 








l\ 




1 \ 




1 \ 




I \ 




\ 




\ 







FIGURE 4o - Lemniscate shield. 



FIGURE 3. - Lemniscate tracks. 



A = 



2 T e 



yielding 



elongations of the link bars intersect. 
The solid lines show the range of action. 

The support force remains nearly uni- 
form over the operating range of the lem- 
niscate shield, while support forces in 
caliper shields increase with the min- 
ing height. The structures of caliper 
shields must be dimensioned accordingly 
to resist the greatest load and may be- 
come very heavy. Higher efficiency of 
the system, uniform support force, and a 
nearly equal span of roof exposure in 
terms of the mining height are the advan- 
tages of the lemniscate shields versus 
the caliper shields. 

The- two legs of this early lemniscate- 
type shield are arranged between the gob 
shield and the base so that the support 
force is introduced into the hinge be- 
tween the canopy and the gob shield, A 
mechanical disadvantage in terms of the 
extraction height reduces the force that 
acts on the canopy hinge. 

Support force and mean load density can 
be estimated by following formulas: 



. 2S e ^^, 
K = f — c~M setting 



A = 2 T e 

« fc C M 



yielding 



A = 



2 S e 



setting 



where S = setting load per one leg, 
kN, 

T = yield load per one leg, kN, 

A = roof support force at the 
canopy hinge, kN, 

C = length of roof bar plus span 
of canopy tip to face, m, 

M = shield spacing, m, 

A^, = mean load density, kN/m^ , 

and e, fg are measured for each work- 
ing height, m. 

Figure 4 is the schematic of a mod- 
ern lemniscate shield introduced in Illi- 
nois in 1978 that has become the standard 



two-leg shield design In the United 
States and abroad (21) » The legs are In- 
serted directly Into the canopy, and the 
gob Is jointed to the canopy rear end to 
eliminate the dead corner where debris 
can accumulate and foul the canopy move- 
ment. A large ram stabilizes this joint 
and provides an adequate load at the tip 
of the canopy. 

Double-telescoping legs provide the re- 
quired range of shield height. The dif- 
ferent support forces in each stage of 
such a leg can be equalized either by an 
internal yield valve in the bottom of the 
piston of the smaller stage or by design- 
ing equal piston areas for each stage 
(fig. 5). 

Efficiency of this design is greatly 
Improved over that of previous ones. The 
support force available at the canopy 
Is 90 pet of the leg force and greater. 
Double-telescoping legs are standing 
nearly straight with a minimum of incli- 
nation within the working height range. 

The resultant magnitude of the support 
force, its location on the canopy, tip 
load, breakoff load, and mean load den- 
sity are estimated by the following for- 
mulas (fig. 4) : 

Ax = S s + N n 



A (x + a) = S e 
S (e - s) - N n 









a 






= 


N 


n + S 


s 






A 




K 




A_ 


(x + 


c) 




FIGURE 5. - Double-telescoping leg. 



A = resultant support force, kN, 



D = A - K 



A =^ 
« CM 



where S = force of two legs, kN, 

N = force of stabilizing ram, kN, 



X = resultant distance from 
joint, m, 

L = length of canopy, m, 

K = tip load, kN, 

D = breakoff load, kN, 



C = length of canopy plus span of 
canopy tip to face, 



M = shield spacing, m, 
Aj, = mean load density, kN/m^ , 
Given: Tip load 

Canopy length 
Force of two legs 



Canopy length plus , span 
of canopy tip to face 

Shield spacing 



K = 

L = 

S = 

C = 

M = 



and e, s, n, a are measured for each 
working height, m. 

An example of evaluation of resultant 
force magnitude and location and mean 
load density follows: 

50 kN 

2.7 m 

2 X 1,600 kN 

2.7 + 0.3 m before the shearer pass 



1.5 m 
0.3 m 
0.8 m 
2.42 m 
2.82 m 

Results: Stabilizing ram moment N n = K (L - c) = 50 x 2.4 = 120 kNm 

S (e - s) - N n 



c = 



s = 



a = 



e = 



Resultant support force 



A = 



Resultant distance from x = 
j oint 

Mean load density before A,, = 
cutting 

Figure 6 is the schematic of a four- 
leg shield with two rear legs supporting 
the gob shield and thus stabilizing the 
joint. These shields provide high tip 
loads and achieve a more even floor load- 
ing than two-leg shields. They are used 
in several mines in Ohio, Pennsylva- 
nia, and West Virginia (fig. 7). Support 
force, tip load, breakoff load, and 
mean load density are evaluated as fol- 
lows (22): 



2 X 1,600 (2.82 - 0.8) - 120 „ ,^^ ,„ 
2742 = ^'^^^ ^^ 



Nn+Ss 120 +2x 1,600 x 0.8 , ^„ 

1 = ?: — ttt:^ = i.UZ m 

A 



2,620 



A 2,620 t._„ ,.^/ 2 

C^ = 3 X 1.5 = ^^2 kN/m2 



A]: c = S^ s 



Ai (b + c) = Si e 



c = 



s b 



ei - s 



1 ^1 



Ai = 



_ ^1 



Si s 



S2 eg ^ 

A2 b ^ 



Aj c 
A^ + A2 



A = Si ei H- S2 eg 
a + b 



10 



T = A 



(a + d) 



D = A - T 

A =-A- 
^ CM 




FIGURE 6. - Four-leg shield. 



where Si = force of two front legs, kN, 

S2 = force of two rear legs, kN, 

Ai = support force resultant for 
front legs, kN, 

A2 = support force resultant at 
the joint, kN, 

a = resultant distance from 
joint, m, 

A = resultant magnitude, kN, 

T = tip load, kN, 

D = breakoff load, kN, 

L = length of canopy, m, 

C = length of canopy plus span 
of canopy tip to face, m. 




FIGURE 7. - Roof shield. 



11 



and 



M = shield spacing, m, 

A,, = mean load density, kN/m^, 

ej, 62, s, b are measured for each 
working height, m. 



Ai Ci + A. 



Figure 8 Is a schematic of a four-leg 
shield where all four legs support the 
canopy. A version of this type, the 
chock shield with straight legs, is the 
preference in National Coal Board mines. 
Table 2 shows that the chock shield, ap- 
plying immediate forward support in the 
one-web-back mode, clearly stands out as 
the roof support with the most complete 
cover and highest load density. 

In the United States V-type shields are 
used in low coal in West Virginia and in 
some thick coalbeds in the West (fig. 9). 

Support force resultant magnitude and 
location and mean load density are esti- 
mated by the following formulas (22) 
(fig. 8): ~ 



Ci = 



Si b 
ii - s 



Co = 



eo - s 



Ai = 



_ Si si 



A2=^ 



a = 



A = 



_ '^l 





Ai 


+ 


A2 




Si 


ei 


+ 


S2 


^2 




a 


+ 


b 




Aw 




A 

C M 






FIGURE 8.- Chock shield. 




FIGURE 9. - V-type shield. 



12 



where Sj = force of two front legs, 
kN, 

S2 = force of two rear legs, kN, 

Aj = resultant support force for 
front legs, kN, 

A2 = resultant support force for 
rear legs , kN , 



and 



a = resultant distance from 
joint, m, 

A = resultant support force, kN, 

ej, e2 , Sj, S2 , b are measured 
for each working height, m. 



METHODS OF IMPROVING GROUND CONTROL ON LONGWALL FACES 



The following requirements are critical 
to achieving and maintaining ground con- 
trol on longwall faces: 

1. Minimizing support delays. 

2. Reducing the span of canopy tip to 
face. 

3. Adequate support force. 

Means to meet these requirements are op- 
eration mode, canopy extensions, advance 
with brushing contact, hydraulic supply, 
hydraulic control function, and floor 
control. 

The one-web-back mode provides a conve- 
nient traveling space and immediate for- 
ward support by advancing the roof sup- 
ports at once after the shearer has cut 
by and before the conveyor is moved up. 
However, the entire shield can be brought 
up closer to the face by operating in the 
up-to-the-conveyor or conventional mode. 
The conveyor is pushed ahead first after 
the shearer pass, and the shield follows. 



Roof support, close to the face, can then 
be applied without delay if canopies are 
equipped with powered extensions. 

The canopy of a shield designed to op- 
erate in the one-web-back mode must be 
longer than that of a conventionally ad- 
vancing shield by at least the width of 
the cutting web, and there is a trend 
toward increasing canopy length to ac- 
commodate wider webs, larger conveyors, 
shearer haulage racks , and even pipes to 
carry material for placing packwalls in 
the gateroads. 

However, a canopy should be designed 
to maintain a stable roof contact during 
shield advance by having a ratio of its 
front portion to its rear portion of no 
greater than 2:1. Front and rear por- 
tion are related to the locus of the sup- 
port force resultant. Canopies with long 
front portions have poor roof contact. 
The extent of unsupported roof is widened 



TABLE 2. - Evolution of roof cover and load density by face support systems 



Face support system^ 



Yield load, 
mt 



Area, 
m2 



Cover, 



m" 



Cover, pet 
of area 



Density at 
yield, mt/m^ 



Before cutting: 

4-leg chock 

6-leg shielded chock...., 

2-leg shield (IFS) , 

4-leg chock shield 

4-leg chock shield (IFS), 
After cutting: 

4-leg chock 

6-leg shielded chock,,,. 

2-leg shield (IFS) 

4-leg chock shield 

4-leg chock shield (IFS) 



100 
260 
325 
450 
450 

100 
260 
325 
450 
450 



3,0 
4,8 
5.1 
5.1 
6.0 

3.5 

5.5 

5.1 

6.25 

6.0 



0.7 

4.0 

4.35 

4.35 

5.74 

,74 
4,32 
4,35 
4.85 
5.74 



23,3 
83.3 
85.5 
85.5 
96.0 

21,1 

78,5 

85,51 

77,5 

96,0 



33,3 
54,2 
63.7 
88.2 
75.0 

28.6 
47.3 
63.7 
72.0 
75,0 



^IFS = immediate forward support. 



Source: Lewis (7, p. 13J . 



13 



because the front of the canopy does not 
touch the roof. 

Canopy extensions can provide the de- 
sired roof contact. Figure 10 shows that 
a canopy divided by a ratio of 2.6:1 
contacts the roof 0.4 m behind the canopy 
tip according to tests conducted at Es- 
sen research facility. Federal Republic 




2.6 2.4 2.2 



2.0 1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 

CANOPY LENGTH, m 




400 

UJ 

q:<v' 300 

ZD E 

CO o 200 

CO \ 

^ Z 100 

'^ 



1 1 1 1 1 1 1 r 

Contact length — 




2.6 2.4 2.2 



2.0 1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 

CANOPY LENGTH, m 




Lever 

FIGURE 10. - Load distribution on a canopy. 



of Germany. Full roof contact can be ob- 
tained by applying an articulated canopy 
extension to the roof with a rather small 
force, e.g., 50 to 100 kN at the tip. 

This type of canopy extension, called a 
flipper, is used with shearers in suffi- 
cient mining height because the retracted 
flipper must clear the shearer. Other 
types of canopy extensions are pushout 
cantilevers of various designs. Some are 
laid on the main canopy. Others slide 
out of the profile so that they can be 
extended even while the shield is set. 
Figure 11 shows a sliding cantilever ex- 
tended with an upswept motion for close 
roof contact in support of a fragile 
stratum. 

When retracted, canopy extensions allow 
an immediate change from the one-web-back 
to the conventional mode to increase the 
mean load density of a roof support by 20 
to 25 pet, if required for roof control 
{9) . Flipper-type extensions can also 
serve as sprags to secure a coal face 
and prevent spalllng of coal. For the 
same purpose, powered sprags are used in 
high coal and are mandatory in the United 
Kingdom and the Federal Republic of 
Ge rraany . 

Automatic operation of pushout canti- 
levers in tandem with the double-acting 
advancing ram has proved to be very suc- 
cessful in plow faces by applying roof 
support close to the face (23). 




FIGURE n. - Upswept sliding canopy extension. 



14 



Advancing a shield while automatically 
maintaining a brushing roof contact keeps 
the roof intact and removes debris from 
the canopy. Compression of a cushion of 
debris may cause excessive yield and, 
hence, bed separation. Friction between 
roof and canopy and between sideplates 
of adjacent shields induces the resist- 
ance that must be overcome during shield 
advance with brushing roof contact. A 
force of 1 mt/m^ brought to bear against 
the roof is adequate to support a 0.4-m 
strata of roof. The height of 87 pet of 
measured roof cavities in German mines 
was less than 0.4 m (19) . 

The hydraulic system on a longwall face 
must have sufficient capacity to coordi- 
nate the speed of shield advance with the 
shearer haulage speed of 6 to 8 m/min and 
to minimize the delay of supporting the 
roof. A pump capacity of 200 L/min is 
average for shearer faces, but rapid ad- 
vance of a face may require a larger 
supply. 

A modern powerpack consists of heavy- 
duty pumps of 56 and 112.5 kW. Working 
pressures of 34 MPa are common. Mixing 
and storage tanks hold 1,000 L of 5 pet 
oil-in-water emulsion. The trend is 
toward single pressure circuits that 
greatly simplify the supply system. Pow- 
erpacks used to be kept near the faces in 
the maingate area and were moved outby 
with retreating faces frequently, but the 
use of stationary powerpacks is on the 
increase. They are located in the main 
line entries from where they can supply 
pressurized fluid through 50-mm pipelines 
to several inby faces without relocation. 

Supports are commonly set with low set- 
ting pressures. The average is 65 pet of 
the rated pressure measured on German 
longwall faces (18) . On a 200-m face, 
pressure losses for a fluid supply of 120 
L/min may range from 3 to 18 MPa accord- 
ing to the hose sizes and the system of 
installation. A straight hose line along 
the conveyor is better in maintaining 
pressure than lines hung from support to 
support. A ring main or duplicate feed 
provides a fluid supply with the least 
pressure loss. 

Jack setters often fail to hold the 
pressure to a shield long enough to bring 
the full available pressure to bear. 



Automatic positive controls achieve the 
full rated working pressure independently 
from the operator. For instance, the 
jack setter initiates the setting to a 
pressure of approximately 12 MPa, at 
which point the automatic control takes 
over and brings the pressure to a rated 
31.5 MPa (23). 

Unidirectional controls of each roof 
support from its neighbor provide shelter 
for the jack setter and keep him or her 
on the upwind side of the ventilation 
current. Figure 12 shows a piano key se- 
lector for adjacent pushbutton control. 
Some roof support systems are controlled 
by adjacent bidirectionally acting valves 
(fig. 13). 

Pilot control systems have been intro- 
duced to replace full-flow hoses between 
supports by one multicore hose carry- 
ing the pilot pressure in 6 to 18 small- 
diameter hoses to make the hosing of the 
support more manageable. However, the 
possibility exists that the small-caliber 
hoses, 2 to 6 mm in diameter, and the 
small bores in the printed gaskets that 
channel the control fluid through the 
valve blocks may be susceptible to infil- 
tration by dust, rendering the system 
inoperable. 

Remote control in batches of 10 units 
is designed to locate the shield operator 
in intake air in both directions of cut- 
ting ( 24 ) (fig. 14). The shearer oper- 
ator initiates the automatic cycle of 
shield movement when he pushes a batch- 
initiating valve attached to a shield. 
The shield operator then activates a se- 
lector valve, and the shield movement 
begins. Each shield, one by one, goes 
through the cycle of advancement. Move- 
ment of a shield starts only after the 
preceding shield has reached full working 
pressure. This is a factor favoring safe 
roof control. Another important safety 
feature is that it takes the action of 
both shearer and shield operator to actu- 
ate the batch movement cycle; in an emer- 
gency, a stop valve can interrupt the au- 
tomatic sequence and bring It to a halt. 

Electrohydraulic controls are an alter- 
native to hydraulic automatic control 
systems and are rapidly gaining accept- 
ance. They are expected to meet the fol- 
lowing objectives (25): 



15 



W fi 




FIGURE 12. - Piano key selector. 




FIGURE 13. - Bi-di rotary valve. 



16 



Ventilation - 




Shield operator 

(remote control: 

Group 4) 



R Step 1, shearer operator restores pressure 
to remote control of Group 4 



j^l [1^ 



Group 2 Group 3 



u, Group 5 Group 6 



Group 4 

-4 Step 2, shield operator initiates advancement 
of Group 4 remotely 

FIGURE 14. - Remote batch control. 



1. Improved roof control on the face 
through an accelerated roof support cycle 
and achieving and maintaining the rated 
setting load in each support. 

2. Keeping shield operators in intake 
air. 

3. Easier assembly, operation, and 
maintenance because most hydraulic con- 
trol hosing is eliminated and connections 
between shields are few. 

4. Better coordination between mining 
and support cycles. 

5. Telemetric monitoring of roof 
support. 

6. Potential of shearer-activated sup- 
port movement. 

Control of the floor is as important as 
roof control. Shields must meet the fol- 
lowing requirements: 

1. The contact pressure exerted on the 
floor shall not exceed 200 N/cm^, 

2. The projection of the resultant 
support force on the base shall be situ- 
ated as far as possible behind the toe of 
the base. 

3. The toe of the base shall be shaped 
to glide over debris. 

Tests are conducted in the laboratory 
to determine floor pressure by measuring 
penetration of a building board plate 
sandwiched between the lower platen of a 
test rig and steel bars placed under the 
shield base. The floor pressure is com- 
puted from the depth of penetration, the 
force applied, and the dimensions of the 
base. 

Results of tests conducted at Essen 
indicated that little difference exists 
between two- and four-leg shields if all 
legs are loaded equally. Generally four- 
leg shields are preferred in coalbeds 




1 00 -_ ^- - _ Mean floor pressure 



120 N/cm' 



SHORT STIFF BASE 




Mean floor pressure 
75N/cm2 



300 L 



LONG ELASTIC BASE 



^ 




100 
200 
300 



Mean floor pressure 
l20N/cm2 



SHORT ARTICULATED BASE 
FIGURE 15. - Floor pressure through different bases. 

above 3 m and below 1.5 m and in soft 
floor. Six-leg shields are recommended 
in coal below 1.2 m. They combine a safe 
and convenient travelway with a mini- 
mum of prop free front and a high load 
density ( 13) . 

Dimensions, shape, and elasticity of 
the base affect the function of a shield 
in adverse floor coriditions (22). The 
top of figure 15 shows a shield with a 
short and rigid base. The shield with 
the maximum floor pressure at its toe 
could sink into a soft floor and ulti- 
mately get stuck, causing major delays. 
The favorable effect of a long elastic 
base is shown in the center part. The 
bottom drawing shows an articulated base. 
The articulated member is called a pendu- 
lum plate, and its length is divided at 
the ratio 2:1. The maximum pressure 
occurs at the end of the short lever 
arm. However, the tip remains unloaded 
so that the shield can overcome obstacles 
and soft uneven ground while advancing. 



17 



Pendulum plates can be mounted on solid 
or divided bases. 

Solid bases have the advantage of ex- 
erting less floor pressure than di- 
vided ones owing to their larger contact 
area. Some come equipped with lifting 
jacks to raise the shield out of a soft 
floor. However, shields with divided 
bases function better in adverse floor 
conditions because each half of the base 
can be lifted individually by its double- 
acting leg. A divided base is also self- 
cleaning in that debris and slack coal 
can pass into the gob through a number of 



open spaces in the base while the shield 
is advanced. 

The tendency of a shield base to dig 
into the floor is counteracted by having 
the double-acting advancing ram mounted 
in an inclined position. The ram acts 
through reverse linkage using relay bars 
to raise the base slightly and to exert 
the full force of the piston end when the 
shield is pulled up to the conveyor. Al- 
so the shield must be designed so that 
the toe of the base is ahead of the pro- 
jection of the resultant support force on 
the base. 



REDUCTION OF SHIELD WEIGHT AND SIMPLIFICATION OF FUNCTIONS 



Table 3 shows that the demands on the 
shield structure have led to considerable 
increases in weight and complexity due to 
many hydraulically controlled functions 
(26-28). According to tests at Essen, 
the weight of the structure is related 
to — 

1. The ratio between closed and ex- 
tended height of a shield. 

2. The inclination of legs. 

3. The magnitude of forces parallel to 
the strata. 

Figure 16 shows the results of tests at 
Essen. The ratio between the collapsed 
and extended shield height determines the 
difference in yield load over the range 
of a shield extension. The difference 



between maximum and minimum load is shown 
as 1,200 kN at an extension ratio of 1:3 
and drops to 350 kN at a 1:2 ratio. The 
extension ratio affects the shield weight 
because the structure must be dimen- 
sioned to carry a large load over a rela- 
tively small part of the entire range. 
Conversely a minor support load due to a 
lesser extension ratio of reduced range 
of height will require a less heavy 
structure. Components such as canopies, 
gob shields, and link bars can be made 
lighter. 

Tests showed that the weight of a four- 
leg shield can be dropped by 31 pet if 
the extension ratio is reduced from 1:3.3 
to 1:2.5. Surprisingly, substituting 



TABLE 3. - Comparison between early and modern shields 



Shield type 


Comparable data and quantities 


Lemniscate as percentage 




Caliper (1975) 


Leraniscate (1982) 


of caliper shields 


Height, cm: 

Closed 


130 
250 

2,200 
1,640 

390 
330 
5 
897 
153 
6.5 


90 

240 

3,240 
2,420 

400 

302 

11 

2,824 

578 

12.2 


NAp 
NAp 

147 


Extended 


Support force at yield, kN: 
Maximum .................. 


Mi nimum 


148 


Mean load density, kPa: 
Maximum. 


103 


Minimum .................. 


92 


Cylinders 


220 


Parts 


315 


0— rings 


378 


Weight mt. . 


188 



NAp Not applicable. 



Source: Hahn (27, p. 157). 



18 




ou 


1 1 1 1 1 
0.6- 1.8 m 


.^ 


^^^-^^ 


^^^25 


/ ^\ 


•> 


/ 0.9-1.8 m \^ 


UJ 


/ >^'^""^**^ \ 


^20 


1 X ^\ \ ~ 


o 


f^r ^V \ 


U- 


Jr ^^ \ 


fe 


Extension ratio \ 
1 to 2 "^ \ 


o 




g: 15 


V. 


ID 


\ 


CO 


Extension ratio 


in 


1 to 3 

1 1 1 1 1 



1.8 1.6 1.4 1.2 1.0 0.8 0.6 
SHIELD HEIGHT(h), m 

FIGURE 16. - Effect of extension ratio on yield load. 

four-leg shields for two-leg shields will 
result in a weight saving of 18.5 pet. 

Considerable inclination of legs is re- 
quired in thin-coalbed mining with the 
consequence that the support force great- 
ly increases in terms of mining height. 
Figure 17 shows an increase from 1,400 kN 
to 2,400 kN when the shield is raised 
from 0.6 to 1.4 m. However, the struc- 
ture must be dimensioned to accept the 




1.4 1.8 0.6 1.0 

SHIELD HEIGHT, m 

FIGURE 17. - Equalization of support force for 
shields with inclined legs. 

maximum force. Equalization to a me- 
dian force of 1,900 kN will result in a 
lighter structure. The equalization will 
be achieved by adjusting the leg pressure 
in terms of the shield height or the in- 
clination of the rear link bars. 

A shield with straight double- or even 
triple-telescoping legs is not free from 
lateral forces exerted by the motion of 
the roof strata from the face to the gob 
and the base movement during setting. 
Figure 18 shows the increase of forces in 
the front link bars of such a shield with 
an extension range from 2.2 to 6m and 
considering a friction coefficient of 0.3 
between canopy and roof rock. The dif- 
ference in tensional load is very high 
and can be equalized by substituting hy- 
draulic cylinders for the front link 
bars. Controlling the force in the hy- 
draulic cylinders by a yield valve limits 
bending moments in gob shield and base 
and thus results in a lighter structure. 

In conclusion, weight reduction is 
achievable by reducing lateral forces 
parallel to the strata by means of hy- 
draulic link bars and regulation of 
the leg pressure to limit peak loads. 
However, it appears that weight reduction 



19 



and design simplification are conflicting complexity 
demands. Weight savings lead to more Germany. 



according to the tests in 



SHIELD DESIGN AND TESTING 



Manufacturers use CADD (computer-aided 
design drafting) (29-30) . The programs 
include assessment of the foundation 
pressure, linkage and pin design, and fi- 
nite stress analyses to generate dimen- 
sions of structural members and amount of 
weld. After solving technical problems, 
microcomputers produce technical specif- 
ications, outline drawings, and prices 
needed for quotations, thus alleviating 
strain on technical resources and deliv- 
ering tenders to customers in minimum 
t ime , 

In the United Kingdom, Testing Proce- 
dures for Powered Support were first is- 
sued in 1966 and updated recently. The 
HSE (Health and Safety Executive) Tests 
include (16) — 

1. Strength and performance. 

2. Stability. 

3. Possibility of abrupt failure of 
components. 

Fh=0.3 Fab Fh limited if Fl= constant 




120 



Hydraulic 
link 



Yield 
valve 



O 



100 



if) 

a: 

-I 60 



LlI 

O 40 

20 



KEY 
F/^t) Support force 
Fh Horizontal force 
Fi_ Link bar force 




F^ol F^-0.5 F^^ 



F|_= constant 

I 



4 6 8 10 

RANGE, m 

FIGURE 18. - Equalization of tensional force 
in link bars by substituting hydraulic cylinders. 



4. Practical design. 

5. Reliability. 

Tests are performed in load-reactant 
frames at the Bretby testing facility 
of the Mining Research and Development 
Establishment. The roof supports are 
exposed to thousands of cycles between 
setting and yield load plus 12 pet. Com- 
mercial approval will be granted to manu- 
facturers after successful laboratory 
testing, underground trials, and National 
Coal Board technical approval. 

In the Federal Republic of Germany, 
statutory examination and approval of 
each prototype roof support is carried 
out in the Material Testing Center of 
Nordrhein-Westfalen State at Dortmund, 
The objective of testing is to assure 
that minimum safety standards are met 
(31) . Testing used to be limited to 
structural members such as legs, can- 
opies, and bases. However, with the ad- 
vent of shields it became necessary to 
subject whole units to dynamic tests, and 
a four-column testing machine was in- 
stalled with a maximum opening of 4.8 m. 
The press produces 6,300 kN vertically 
and 2,000 kN hotizontally (fig. 19). The 
test program includes reliability of hy- 
draulic controls. Special yield valves 
designed to shed load quickly in the 
event of a rockburst and thus prevent 
structural damage to legs are also tested 
in the press, which can retract the upper 
platen at a velocity of 0.5 m/s. 

The test facility at the Essen research 
center is mining research and development 
related and offers the opportunity to 
manufacturers to test their designs un- 
der simulated underground conditions. A 
large test frame was built in 1963. 
Whole roof support units can be subjected 
to convergence and lateral thrust through 
the movement of an inclined top platen 
against a floor platen in opposite 
directions (19). 



20 



4- 



KEY 

Gages 

/ through 5 



iR 




J*M 



4 '^5 






SIDE VIEW 



BASE, BENDING 



Not to 
scale 



I 



BASE, TORSION 



H 



I 



'/3 -bearing near toe 
PLAN VIEWS FROM /I 

FIGURE 19. - Schematic of testing bending and tor- 
sional loads in the structure. 

In 1975 a 5,900-kN press was completed 
to accommodate an urgent need for testing 



shields that came on the German mining 
scene in increasing numbers. The tests 
include forces exerted on roof and floor, 
the support resistance, bending moments 
in the structural members, penetration 
of the base into the floor, reliability 
of shield structure under load cycling, 
function and operating mode of the 
canopy, angular positioning of canopy 
and gob shield, width and height of 
travelway, behavior on uneven ground, and 
sideways mobility of the double-acting 
advancing ram. 

In the United States the Bureau of 
Mines Mine Roof Simulator at the Pitts- 
burgh Research Center is the most power- 
ful research facility of its kind in the 
world. It is a computer-controlled elec- 
trohydraulic press with a maximum opening 
of 4.9 m. It can simultaneously exert 
13,300 kN vertically and 7,100 kN lat- 
erally to simulate strata conditions to 
which roof supports could be exposed (32- 
33 ) . The Mine Roof Simulator can play a 
role in domestic fabrication of roof sup- 
ports to find flaws in design so that 
manufacturers may know where to direct 
efforts for improvement. 

The Bureau of Mines is collecting field 
data from instrijmentation to measure leg 
and canopy cylinder pressures by trans- 
ducers and strain in lemniscate link bars 
by strain gages. These data are used to 
determine the resultant load vector on a 
shield by its parameters: magnitude, lo- 
cation on canopy, and inclination. These 
parameters provide design information 
to assess internal and external forces. 
Analysis of field data can be achieved by 
feeding the data into the computer of the 
Mine Roof Simulator to obtain a simulated 
load profile. The objective of this ef- 
fort is to advance understanding of sup- 
port behavior as a first step in improv- 
ing the design (34). 



DUST CONTROL AND VENTILATION 



Automatic sprays may be mounted on the 
canopies to control dust originating from 
the gob during shield advance, and the 
working space is sealed against dust by 
sideplates fitted between the shields and 
kept tight by springs and hydraulic rams 



(fig. 20) . Figure 21 is a chart indicat- 
ing the experience with several types of 
dust seal arrangement in the Federal Re- 
public of Germany. The sideplates are 
also used to steer and align adjacent 
shields against each other. This is most 



21 




DUST SEAL TYPE 



FIGURE 20. - Side shield. 

important In undulating ground and in 
pitching strata. 

High air velocities in the restricted 

longwall cross-sectional area can cause 



Laid upon 



Underslung 



SHIELD ALIGNMENT 



Up to 18* pitch 



Undulating roof 



Most frequent design 



Little funnel effect 



Strong funnel effect 



FIGURE 21. - Effect of dust seals. 

physical discomfort to miners and initi- 
ate aerodynamic dust entrainment. Re- 
search in dust elutriation carried out in 
England by the National Coal Board indi- 
cated that the threshold velocity lies at 
about 135 m/min, above which the aerody- 
namic entrainment increases very rapidly. 
The ventilation cross-sectional area 
(Q) is a function of mining height ac- 
cording to an empirical formula developed 
in Europe (35) : 



Q = 3.75 (M - 0.3) for chocks 
and four-leg shields, m^, 

Q = 3 (M - 0.3) for two-leg 
shields, m^, 



or 



where M = mining height, m. 

Obviously, in terms of face ventilation, 
faces with two-leg shields are at a dis- 
advantage relative to those with chocks 
or four-leg shields. 



THE ROLE OF THE BUREAU OF MINES IN DEVELOPMENT 
OF LONGWALL ROOF SUPPORT TECHNOLOGY 



The Bureau of Mines has been an ini- 1980 
tiator of longwall roof support technol- 
ogy, and the following milestones mark 
the Bureau's contribution to its rapid 
progress: 1981 

1975 One of first two shield faces in 
the United States installed in the 

York Canyon Mine, New Mexico. 1982 

1976 The first lemniscate-type shields 
installed in the Old Ben No. 24 
Mine, Illinois. Subsequently the 
company purchased 10 more longwall 
faces for operation in the Illinois 
division. 



The Mine Roof Simulator at Bruce- 
ton, PA, and related field data 
acquisition, as described under 
"Shield Design and Testing." 
The first longwall in a steeply 
pitching coalbed in Snowmass Mine, 
Colorado. 

The top slice in a multilift ex- 
traction of a thick coalbed. This 
is an advancing face, unique in the 
United States, in the Dutch Creek 
No, 2 Mine, Colorado, 



22 



A thin-seam mining project was shortlived 
and terminated owing to unsuitable strata 
conditions. 

DEMONSTRATION OF SHIELD-TYPE 
LONGWALL ROOF SUPPORTS 

The Bureau of Mines entered into a 
cost-sharing agreement with Kaiser Steel 
in 1974 to demonstrate longwall mining 
with shield supports in the Mesa Verde 
coal basin in New Mexico. At Kaiser's 
York Canyon Mine, ongoing longwall mining 
with chocks had met with severe roof con- 
ditions. Wire mesh had to be stretched 
over the canopies to contain debris drop- 
ping out between the roof supports. Pro- 
ductivity under these circumstances was 
very low. 

The Hemscheidt^ 320 HSL caliper-type 
shields selected by Kaiser and Bureau en- 
gineers in 1974 proved to be most effec- 
tive in sheltering the working place from 
debris and dust. The shield face that 
began operation in 1975 was one of the 
first longwall faces supported by shields 
in the United States, The shields pro- 
vided adequate roof control for the ex- 
traction of three panels and were then 
transferred to Kaiser's Sunnyside Mine in 
Utah, where in 1982, the shield face at- 
tained a world record longwall production 
of 18,497 mt of raw coal in 24 h. Today 
caliper shields are an obsolete design 
and are no longer fabricated. 

Figure 22 is a schematic of the se- 
lected two-leg caliper shield. The ver- 
tical extension ranges from 1,50 m to 
3.5 m, of which half is hydraulic and 
half is by mechanical extension members 
applied to the legs. The 2^n-wide exten- 
sion range is achieved by having two po- 
sitions of the legs on the base and also 
two positions for the joint between the 
gob shield and the base. The legs are 
single telescoping. 

The shield weighs 9 mt and is 4.3 m 
long. The powerpack supplies a hydrau- 
lic working pressure of 34 MPa, and the 
shield is designed to yield at 41 MPa. 

■^Reference to specific products does 
not imply endorsement by the Bureau of 
Mines . 




FIGURE 22. - Caliper shield 320 HSL. 

Thus, the working pressure equals 83 
pet of the yield pressure to maintain a 
strong thrust against the roof strata im- 
mediately after exposure and to prevent 
bed separation. 

A 0,5-m pushout cantilever compensates 
for the widening span between canopy tip 
and face due to the circular motion of 
the canopy tip of a caliper shield when 
the shield is raised. 

The two legs are arranged between the 
gob shield and the base so that the sup- 
port face is introduced into the hinge 
between canopy and gob shield. The force 
in the two legs can exert a thrust of 
2,620 kN and yield a load of 3,140 kN, 
The mechanical disadvantage due to the 
shield geometry is a function of the ex- 
traction height and reduces the force 
that acts on the canopy hinge. The 
shield is operating in the one-web-back 
mode to provide a convenient travelway. 
Mean load density at the yield point 
ranges from 345 to 505 kPa over the ef- 
fective extraction range from 1.70 to 
3.35 m. 

The canopy is designed to maintain a 
stable roof contact during the shield 
advance by the action of two rams that 
swing it against the roof and by having a 
ratio of front portion to rear portion of 
canopy of no greater than 2. The canopy 
tip exerts a load of 49 kN at yield. 

The divided base is self-cleaning and 
the double-acting advancing ram, acting 
through reversed linkage, is mounted in 
an inclined position to counteract the 



23 



tendency of the shield to dig into the 
floor. Also the shield is designed so 
that the toe of the base remains ahead of 
the projection of the canopy hinge joint. 

The shields were first controlled di- 
rectly. However, during the first face- 
to-face move the controls were changed to 
adjacent operation to protect the shield 
setter. The shield can be advanced while 
automatically maintaining a soft roof 
contact to keep the roof intact after ex- 
posure and wipe debris off the canopy. 

Dust originating from the gob during 
shield advance is controlled by automated 
sprays mounted on the canopies. The 
working space is sealed by sideplates 
fitted between the shields and maintained 
by springs and hydraulic cylinders. 

DEMONSTRATION OF LONGWALL MINING 

The Illinois coal basin holds signifi- 
cant reserves and is one of the important 
coal producing provinces in the United 
States. Most underground coal mines in 
Illinois use room-and-pillar methods with 
the result that the average recovery of 
coal approximates only 50 pet, roof con- 
trol is difficult, and productivity needs 
to be improved. 

After previous attempts at longwalling 
in Illinois using chock-type supports had 
failed, the Bureau of Mines entered into 
a cooperative agreement with Old Ben Coal 
Co. in 1975 to demonstrate longwall min- 
ing in the Herrin No. 6 Coalbed, As a 
result of a premining review. Mine No. 24 
near Benton, IL, became the demonstra- 
tion site, and a roof support system was 
specified (9^). Extraction of the first 
longwall panel indicated that the roof 
could be controlled by shield-type sup- 
ports. Consequently Old Ben purchased 
equipment for 10 additional longwall 
faces start-ing in 1978. Longwall mining 
technology greatly benefitted from Old 
Ben's expertise. 

The Thyssen RHS 18/30 shields selected 
by Old Ben and Bureau engineers in 1975 
were the first lemniscate-type shields in 
the United States. The premining study 
recommended a mean load density at yield 
of 862 kPa in the closed position and a 
minimum of 0.15 m of floor coal to remain 



intact, owing to a soft underclay that 
fails at 210 N/cm^ when wet. 

Figure 23 includes a side view of the 
shield in the one-web-back position; the 
shearer and conveyor are also sketched 
to indicate relative position. Vertical 
travel ranges from 1.8 to 3 m in one 
stroke without the use of extension mem- 
bers so that cavities can be supported 
quickly. Owing to the lemniscate gear, 
the span between canopy tip and face re- 
mains 0.3 m over the entire hydraulic 
range to provide a safe clearance between 
the rotating shearer drums and the canopy 
tip. The 5-m-long shield, weighing 15.5 
mt , was the heaviest roof support in the 
United States in 1975. The powerpack 
supplies a hydraulic working pressure of 
30 MPa, and the shield was designed to 
yield at 38 MPa. Thus, the working pres- 
sure equals 80 pet of the yield pressure. 

The two legs are arranged between the 
gob shield and the base so that the sup- 
port force is introduced into the hinge 
between the canopy and gob shield. Ex- 
treme force is concentrated in the 
legs that together can exert a thrust of 
4,600 kN and yield a load of 5,830 kN. A 
mechanical disadvantage due to shield 
geometry is a function of the extraction 
height and reduces the force that acts on 
the canopy hinge. At 2.15-m extraction, 
the shield can exert a thrust of 3,600 kN 
and sustain a load of 4,550 kN. 

Canopy extensions are of the articu- 
lated type, called flipper. They are 0.6 
m long to accommodate a 0.6-m cutting web 
and, when extended, allow a one-web-back 
operation of the shield. However, the 
entire shield can be brought 0.6 m closer 
to the face and operate in the up-to-the- 
conveyor mode by lowering the flipper. 
In the closeup position the shield is 
rated to yield at a mean load density of 
1,200 kPa before the shearer pass and 960 
kPa after the shearer has cut by. 

By design, the canopy maintains a sta- 
ble roof contact during the shield ad- 
vance because a ram pushes it against the 
roof, and the ratio of its front portion 
to its rear portion is not greater than 
2. The tip of the canopy extansion sus- 
tains a load of 36 kN. 



24 



■'rr-".'^.-v. 




'^^^^^^^^^^^^^^^w^^^^^^m^;^w^^^^ 



SIDE VIEW 



FRONT VIEW 



-^^ 




PARTIAL TOP VIEW 
OF BASE 

FIGURE 23. - Lemniscate shield 18/30. 



The base is divided and self-cleaning. 
Mounting the double-acting advancing ram 
in an inclined position corrects the 
tendency of the shield to bury into the 
floor. Owing to reverse linkage, the 
full force of the ram piston end is ap- 
plied to advance the shield. 

The shield is controlled from its 
neighbor to protect the shield setter. 
The shield can be advanced while automat- 
ically preserving a soft roof contact to 
keep the roof intact after exposure and 
wipe debris off the canopy. Automated 
sprays mounted on the canopy and seals by 
sideplates fitted between the shields and 
maintained by springs and hydraulic rams 
provide control of dust originating from 
the gob during shield advance. 

The Thyssen RHS 18/30 shield, when se- 
lected in 1975, was one of the best roof 
supports available and provided adequate 
roof control and dependable operation 
(21) . The experience gained on the first 
longwall faces in Mine No. 24 led to the 
concept of an improved shield type, the 



Thyssen RHS 12/30 and the Hemscheidt 
G-520-12/28 (fig, 24), Improvements in 
the shield geometry added up to a more 
efficient introduction of force from the 
legs into the canopy. At an extraction 
height of 2.1 m, the 1975 shield trans- 
mits 78 pet of the axial leg force into 
the roof, v/hile the modern shield exerts 
96 pet of the leg force. The following 
improvements were made: 

1, The legs are inserted directly into 
the canopy, 

2, The rear end of the canopy is 
jointed to the gob shield to eliminate 
the dead corner where debris could accu- 
mulate, impeding canopy movement, 

3, The lemniscate links are straight, 
eliminating expensive fabrication. The 
rear links no longer need the "clamshell" 
protective plate, 

4, The shields are collapsible to 1.2 
m of height for movement under low clear- 
ance. Double-telescoping legs provide a 
range of 1.3 to 3 m of vertical travel. 



25 





FIGURE 24. - Lemniscate shield 12/30. 



5. The cutting web increased from 0.6 
to 0.75 m to improve productivity, and 
therefore, the flippers were lengthened 
to 0.75 m. 

6. The flippers can be lifted as much 
as 20° up into the roof to provide an up- 
swept canopy extension for close roof 
contact in adverse conditions. 

7. When the flippers are retracted 
and folded under the canopy, clearance 
between them and the shearer body is 
increased. 

8. A large ram stabilizes the joint 
between the canopy rear end and the gob 
shield. With a force of 651 kN the ram 
provides a load of 72 kN at the tip of 
the extended flipper. 

9. The gob shield is shorter and 
lighter. 

10. The mean floor pressure is only 
183 N/cm^ because the effective floor 
contact area is large and the resultant 
roof load transmitted to the floor is lo- 
cated more than 1 m behind the toe of the 
base. 



11. Three shields in each maingate are 
equipped with "pendulum plates," articu- 
lated bases that aid in overcoming soft 
or uneven floor and obstacles. These 
three shields have longer canopies to 
protect the conveyor drive (36). 

12. The modern shield weighs 13 rat, 
2.5 mt less than the early lemniscate 
shield, owing to its lighter structure as 
a result of improved geometry. The lower 
weight and the collapsed height of 1.2 m 
greatly facilitate installation, recov- 
ery, and transportation of the shield 
and reduce cost of face-to-face moves ac- 
cordingly. With flippers retracted the 
shield fits a 4.75-m-long shaft cage deck 
and can easily be turned around in the 
restricted space of a recovery area. 

13. A single pressure hydraulic system 
simplifies hosing of the shields and 
maintenance. 

Specifications of the latest shields ac- 
quired in 1981 are listed in appendix B. 
Similar shield types with or without can- 
opy extensions have become the standard 



26 



for modern two-leg shields in extractions 
ranging from 1.5 to 3m in the United 
States and abroad. 

LONGWALL MINING IN STEEPLY 
DIPPING SEAMS 

Mobile room-and-pillar mining equipment 
works efficiently in strata pitching less 
than 10°. The maximum pitch rubber-tired 
equipment can negotiate is 22°. Also 
ground control problems in pitching coal- 
beds become intense and miners' safety 
critical owing to adverse pillar loading 
and lateral shifting of the overlying 
strata. Resource recovery becomes unac- 
ceptably low. Therefore, in Europe, mod- 
ern mechanized longwall methods using 
shields were adapted to strata pitching 
more than 22° and up to 60°. Mechaniza- 
tion of very steep coalbeds pitching more 
than 60° has been attempted by forming a 
diagonal front. 

In the United States pitching coalbeds 
occur in the West and in the Pennsylvania 
anthracite province. In Colorado alone, 
the reserve base of coal recoverable in 
pitching coalbeds under less than 900 m 
of overburden is estimated to amount to 

1.4 Gmt (27). 

The Bureau of Mines recognized the need 
of introducing modern longwall mining 
methods to steeply pitching coalbeds to 
enhance worker safety and health, produc- 
tivity, and recovery of the valuable re- 
source, and entered into a cooperative 
agreement with Snowmass Coal Co. in 1979 
to demonstrate longwall mining a coalbed 
2.1 m in height and pitching 30°. 

For roof support on a shearer face, 
Snowmass selected Hemscheidt two-leg 
lemniscate-type shields. The legs sup- 
port the canopy, and each leg yields at 
1,560 kN. The shield can be set with 
force of 2,490 kN and is rated to yield 
at a mean load density of 718 kPa at the 
extracted mining height (fig. 25) . The 
legs are double-telescoping and double- 
acting, and the shield can extend from 

1.5 to 3.4 m. The shields are used in an 
up-to-the-conveyor bidirectional opera- 
tion mode and, therefore, are equipped 
with 0.75-m pushout cantilevers and face 
sprags to provide immediate support to 
the face after the shearer pass and to 



prevent spalling of the face coal. The 
cantilever is pushed out of the profile 
of the canopy and thus can be extended, 
while the shield is set against the roof. 

The shields are grouped in three-shield 
sets, which the manufacturer called 
Troika with the Russian three-horse sled 
in mind (38). The Troika sets are self- 
advancing independently from the face 
conveyor, using a 4.5-m-long floor beam 
to accomplish the advance. The center 
shield of each Troika set is rigidly at- 
tached to the floor beam. (After the 
center shield is lowered from the roof, 
the floor beam is advanced by the double- 
acting rams of the two outer shields and 
pulls the center shield a step forward. 
The center shield is then set against the 
roof and holds the floor beam in the for- 
ward position. The downdip shield is 
lowered next, pulled up to the beam by 
its own double-acting ram, and set. Low- 
ering, advancing, and setting the updip 
shield complete the move.) An entire 
Toika set can be steered uphill or down- 
hill by the action of aligning rams that 
maneuver the center shield laterally. 
The floor beam transmits this movement to 
the flanking shields, turning the set. 
Each Troika set is controlled from the 
lower shield of the upslope unit. 

The face conveyor is pushed by double- 
acting rams connected to the topmost 
shield of a Troika set but not attached 
to the conveyor. The conveyor cannot be 
retracted. An in-face anchorage system 
holds the conveyor in place to overcome 
downhill creep due to its own weight and 
the impetus of shearer travel. The an- 
chorage system consists of rams attached 
to every other Troika set. The rams are 
hitched to the conveyor by 381- by 89-mm 
chains. The anchorage must be released 
for each Troika advance. For uphill mo- 
tion of the conveyor, all anchorage rams 
along the face are pressurized. Downhill 
motion is initiated by releasing as many 
as half of all the anchorage rams . 

The maingate end Troika is operated in 
the one-web-back mode to accommodate the 
conveyor drive. The shield line does not 
extend into the tailgate, which is sup- 
ported by fiber-reinforced concrete crib- 
bing and single hydraulic props. 



27 




FIGURE 25. ~ Snowmass shield. 



In the Federal Republic of Germany, 
steeply pitching coalbeds form 15 pet of 
the total reserve base, but only 9 pet of 
the production comes from this category. 
The Troika principle, originally devel- 
oped in Germany and used on the Snowmass 
face in the United States , has been aban- 
doned (39). Each shield is connected to 
the conveyor by its double-acting ram as 
in flat seams because the 4.5-m Troika 
floor beam did not accommodate well to 
adverse floor conditions and accumula- 
tions of debris. Also keeping the face 
straight and controlling the roof when 
traversing strata discontinuities such as 
faults proved to be impractical with the 
Troika approach. 

Figure 26 is a schematic of a shield 
with three side rams to adjust alignment 
of the units. In-face anchorage of the 
panline is achieved by cylinders hitched 
to the conveyor to apply tension to the 
system (fig. 27). 




p 1 Aligning 
^ [ forces 




SIDE VIEW 




PLAN VIEW 

FIGURE 26. - Aligning shield in pitching strata. 



28 




FIGURE 27. - Tensional in-face anchorage. 



LONGWALL MINING OF THICK SEAMS 

Three-fourths of the U.S. coal reserve 
base lies west of the Missippi River. 
There are important reserves of bitumi- 
nous and subbituminous coal in Colorado, 
New Mexico, Utah, and Wyoming that can 
only be mined by deep methods. Accord- 
ing to estimates identified reserves in 
the 3- to 6-m range within these States 
amount to 29 Gmt of bituminous and 70 Gmt 
of subbituminous coal (40). 

A thick coalbed is defined as one that 
falls beyond the range in which produc- 
tivity can be achieved with room-and- 
pillar mining methods. The threshold 
lies at approximately 3.5 m of thickness. 
Wherever mining of thick western coalbeds 
in the 3.5- to 5.5-m range and pitching 
less than 20° is being attempted, con- 
tinuous or conventional room-and-pillar 
methods seldom extract more than 2.5 m of 
mining height in a bottom slice; there is 
an irretrievable loss of more than 70 pet 
of the resource, entailing the hazard 
of spontaneous combustion. Therefore, 
the development of mining methods that 
provide a safe and efficient operation, 
as well as optimal recovery of the valua- 
ble resource of low-sulfur fuel with ac- 
ceptable environmental hazard, is in the 
national interest. 

In Europe and Japan, thick coalbeds are 
extracted by mechanized longwall mining, 
and extraction methods fall into the fol- 
lowing categories: 



1. The full-face or single-pass system 
with extraction of the full seam thick- 
ness in one single lift. Such mining in 
a 4.5-m coalbed takes place in the east- 
ern portion of the Ruhr District, and 
mining of a 5.5-m seam is scheduled for 
the near future. The longwall roof is 
supported by shields with wide vertical 
ranges and operating in a one-web-back 
mode. The coal is extracted with double- 
drum shearers designed to operate in 
thick coal. In the United States a long- 
wall setup capable of extracting coal to 
a height of 4 m in a single pass has been 
operating in Wyoming since 1981. 

2. The multilift system in ascending 
or descending order for an extraction in 
excess of 6 m. Ascending slicing, which 
requires hydraulic or pneumatic stowing 
of the mined-out lift, is practiced in 
Poland. Mining of lifts in descending 
order calls for preparation of an artifi- 
cial mat or leaving a parting for roof 
formation between slices and is performed 
in Japan, Hungary, England at Daw Mill 
Colliery, and the United States at Mid- 
Continent's Dutch Creek Mine (41) . 

3. The sublevel caving or draw system 
in 6- to 15-m coal, which is practiced at 
Blanzy in southern France, at Velenje in 
Yugoslavia, and in Hungary. 

Single-Pass Mining 

Application of the single-pass system 
was addressed by the Department of Energy 
through a contract, "Assessment of the 
Single Pass Thick Seam Longwall Mining 
Method," awarded to Ketron, Inc., in 
1979. The technical approach to this 
study included the type of roof support 
to be installed (39). 

Compared with the other thick-seam min- 
ing methods, the single-pass system has a 
potential of higher productivity and im- 
proved coal recovery. Simplified face 
formation without artificial roof or 
floor provides a faster rate of face ad- 
vance. Gateroad maintenance is easier. 
Problems arising from high liberation of 
methane are more controllable. Improved 
coal recovery diminishes the hazard of 
spontaneous combustion. Potential disad- 
vantages of the single-pass system are 
bulky, and costly roof supports and 



29 



spalling of coal from the face. Per- 
sonnel protection and disposal of large 
pieces of coal or rock must be part of 
the design. 

Figure 28 shows a two-leg shield that 
can be extended from 2.2 to 6m with 
triple-telescoping legs and hydraulic 
lemniscate linkage bars. Substituting 
hydraulic cylinders and yield control 
valves for lemniscate bars accomplishes 
limitations of lateral loads in an ef- 
fort to reduce the weight of the struc- 
ture. The shields are equipped with 
pushout cantilevers and face sprags. 
Such shields are installed in the West- 
falen Mine in the Ruhr District. 

The shields are to be used in a one- 
web-back two-bench mining method (fig. 



29). By first cutting the top bench and 
advancing the roof support and then ex- 
tracting the bottom bench with the face 
sprags set against the top coal, slough- 
ing of the face coal can be reduced to 
manageable proportions. Most thick-seam 
longwalls in Germany are operated on the 
advance by either driving the gateroads 
ahead or profiling them with the face. 

Multilift Working 

In the United Kingdom the 7-m Warwick- 
shire Thick Coalbed at Daw Mill Colliery 
near Coventry is extracted in two slices, 
the top one on the advance and the bottom 
one on the retreat (42) (fig. 30). The 
top slice face is 250 m long, while the 






















.•# 




FIGURE 28. - Two-leg high-seam shield. 



30 



'////////A ^//{/////A ^ 



u 



\^=M 



u 



u 



z^ 



Roof support Top coal cut 1st Start support 

advances, bottom 1 web back 

bench still present 



////////// y///////// 




u 



u 



Conveyor advance 



'77?/, 

Face sprags set 
before cutting 
bottom bench 



FIGURE 29. - Two-bench mining method. 



Mudstone --^^^^r^--.^^=T^.^^ 



Coal 




Advancing face 
in fop secfion 



' ^ Refreofing face 

^ in bottonn secfion 



y r- Nof fo scale 

SEAM SECTION 




Z±3 



PLAN VIEW 



n/t/// ////>/ /ii -T-in, 
SECTION 



FIGURE 30. - Muitilift syster 




FIGURE 31.- Chock shield. 



bottom slice face, to be mined later on 
retreat, will be shortened so that the 
bottom gateroads will be placed in the 
destressed strata within the envelope 
of the pressure arch generated by the top 
slice extraction. Currently the top 
slice is 3.6 m in height, and the roof 
is supported by chock shields, which are 
equipped with face sprags; the shields 
weigh 12.5 mt each and are installed on 
1.5-m centers (fig. 31). Mining plans 
call for a 4.5-m extraction in the 
future. 

In the United States, Mid-Continent's 
Dutch Creek Mine longwall is operated 
on the same principle as the Warwick- 
shire face at Daw Mill in the United 
Kingdom, In 1979 the U.S. Government en- 
tered into a cooperative agreement with 
Mid-Continent Resources to demonstrate 
extraction of the 8.5-m Coal Basin Seam 
in two slices (43). Currently the top 



slice face extracts 3 m of coal on the 
advance. The 245-^-long face was the 
longest in the United States in 1982. 

There are 162 Hemscheidt 2-leg shields 
installed on 1.5-m centers at Dutch Creek 
(fig. 32). The shields extend from 2.4 
to 3.6 m, and the double-acting advancing 
rams provide a 1-m-deep web. The two 
double-acting double-telescoping legs 
yield a load of 455 mt. The shields are 
equipped with pushout cantilevers that 
are extendable from to 1.5 m under full 
load. This feature, unique in shield de- 
sign, is intended to provide control of 
the roof immediately after the shearer 
pass under the most adverse conditions. 
With a friction coefficient of 0.3 be- 
tween rock and steel and 0.1 between 
steel parts, or a total friction coeffi- 
cient of 0.4, it takes a 1,320-kN ram to 
extend the cantilever under full load 
(41). 



31 




FIGURE 32. - Mid-Continent shield. 



The canopy extension offers the options 
of one-web-back or up-to-the-conveyor 
modes. Immediate forward support can be 
provided either by pushing out the canti- 
lever or by pulling the whole shield up 
to the conveyor. A face sprag can be 
extended from the pushout cantilever to 
prevent spalling of the face coal. 

The Mid-Continent multilift demonstra- 
tion takes place in an outburst-prone 
coalbed, and a number of shield legs were 
damaged when a violent coalburst occurred 
in the floor in 1983. Therefore, special 
rapid-load-shedding valves had to be sub- 
stituted for the yield valves that came 
with the shields. 

The sudden impact of a roof rapidly 
sinking with a velocity of 1.5 to 2.5 
m/s releases energies that cannot be 
sustained by the legs. The interior hy- 
draulic pressure rises rapidly and far 
exceeds the design pressure so that 
the legs burst open before the ordinary 
yield valves can shed the load. Burst- 
proof props were first developed for the 
South African gold mines, which have been 
plagued by rockbursts ever since they 
have been in operation. In the special 



Mid-Continent valves, the ordinary yield 
valve of a roof support leg is supple- 
mented by a coalburst valve. If the in- 
terior pressure in a leg exceeds the 
yield pressure, Belleville springs will 
be compressed, causing large ports to 
open and release the hydraulic fluid 
quickly. 

Sublevel Caving 

Sublevel caving as practiced in France 
has not found any application in the 
United States or Canada. Recent reports 
describe four-leg roof shields developed 
by the French that accommodate two ex- 
traction and haulage systems (44) . How- 
ever, ground control may be problematic 
even in favorable strata conditions. 
Coal recovery may be far from complete, 
and the remaining coal is prone to spon- 
taneous combustion, requiring nitrogen 
infusion. 

THIN-SEAM LONGWALL MINING 

An estimated 44 Gmt, or 29 pet, of the 
coal reserve base to a depth of 300 m in 



32 



the Eastern United States falls into the 
0.7- to 1.1 -m range (45), but a much 
smaller proportion of production (10.8 
pet in 1975) comes from this range. Of- 
ten not recovered as the consequence of 
selective mining, this coal is a source 
that will become increasingly important. 
Mining thin coalbeds in sequence from top 
to bottom in the strata profile makes use 
of what resources are available and ex- 
tends the life of mines that age economi- 
cally as extraction moves away from open- 
ings. Another advantage of such planned 
mining is a better control of rock mass 
behavior and methane emission. At pres- 
ent, all active thin-seam longwalls are 
located in the Eastern Coal Province, 
and a number of operations have been sus- 
pended owing to the erosion of the market 
for metallurgical coal. 

The disproportion between production 
from and available reserves in thin seams 
is also typical for Europe. For in- 
stance, in the Federal Republic of Ger- 
many 50 pet of coal reserves occur in 
less than 1.30 m of thickness, but only 
11 pet of the total production comes from 
this category ( 46 ) . 

Shields designed for thin-seam coalbeds 
must meet enough of the following re- 
quirements to provide a satisfactory 
tradeoff (47) : 

1. They must be collapsible to 0.55 m 
for transportation in low entries. 

2. They must have a wide hydraulic 
range of mining height with double- 
telescoping legs. 

3. They must have a 
capacity. Note that 



large support 
a mechanical 



disadvantage, caused by the inclination 
of the legs, reduces the vertical force 
exerted against the roof, particularly in 
low mining height. 

4. To control a fragile roof canopy, 
extensions cannot be used owing to space 
limitation in 1-m and less extraction. 
Therefore, an up-to-the-conveyor mining 
mode is adopted to minimize roof expo- 
sure. This mode is often applied in con- 
nection with crawl pans. 

5. Crawl pans should not be narrower 
than 0.6 m so that injured persons on 
stretchers can quickly be moved out of 
the face area. A travelway width of 0.60 
m is statutory in the Federal Republic of 
Germany. 

6. To follow floor variation, the 
bases often are equipped with pendulum- 
type skids. 

7. A divided base through which de- 
bris accumulated in the travelway can be 
passed into the gob is desirable. 

Several types of shields were de- 
signed to meet these desirable objec- 
tives , including — 

1. Two-leg shields. 

2. Four- and three-leg shields with 
legs arranged in an X-fashion with one or 
two legs supporting the gob shield and 
thus stabilizing the joint between gob 
shield and canopy (fig. 33). 

3. Six-leg shields to combine a safe 
and convenient travelway with a mini- 
mum of propf ree front (fig. 34) . These 
shields are used on shearer faces, while 
the other two categories often operate in 
connection with plows (figs. 35-36). 



CONCLUSIONS AND SUMMARY 



Shields were introduced to U.S. long- 
wall mining in 1975, and in only 9 years 
their innovative development has greatly 
advanced technology. A more favorable 
accident experience has been a welcome 
additional benefit. 



Mine operators who introduce longwall 
mining must select roof supports to op- 
timize performance under site-specific 
strata conditions and mine design re- 
quirements. Their decision making will 
be assisted by the concepts of load 



33 



•V;.4=^l.,l 




FIGURE 33. - X-type shield. 




FIGURE 34.- Six-leg shield. 



34 




FIGURE 35. - Plow face schematic. 




FIGURE 36. - Plow face. 



35 



prediction, factors contributing to 
ground control, and related technology 
described in this report. Information 
presented herein on recent developments 
in advancing roof support technology 
should assist in the development of spec- 
ifications to be issued to prospective 
bidders; such specifications address de- 
sign of roof support components, hydrau- 
lic capabilities, and related technology 
and safety factors. Properties of a two- 
leg shield that was selected through such 
selection criteria in 1981 are listed in 
appendix B, 



Currently, most roof support structures 
come from the United Kingdom and the 
Federal Republic of Germany; appendix C 
lists only one domestic producer. How- 
ever, a trend to improve hydraulic com- 
ponents and fabricate and assemble the 
structures here in the United States is 
noticeable. The high cost of shipping 
entire units from Europe, including in- 
surance and custom duties that amount to 
10 to 15 pet of the unit price, is a fac- 
tor in favor of domestic assembly. 



REFERENCES 



1. Kundel, H. Die Strebtechnik in 
Deutschen Steinkohlenbergbau in Jahre 
1983 (Longwall Face Technology in the 
German Coal Mining Industry in 1983) . 
Gluckauf, V. 120, No. 11, 1984, pp. 669- 
685. 

2. Gross, M. A. 1977 Census of Long- 
wall Installations, Off the Wall, 
Huwood-Irwin, Aug, 1978, 6 pp. 

3. . (Dep, Energy). Private 

communication, 1980; available upon re- 
quest from E. A. Curth, BuMines , Pitts- 
burgh, PA. 

4. . Census of Longwall Instal- 
lations. Coal Age, v. 85, Dec. 1980, 
pp. 89-101. 

5. Sprouls , M. Longwall Census '82. 
Coal Min. and Proc. , v. 19, Dec. 1982, 
pp. 43-47, 50-53, 56-69. 

6. . Longwall Census '83. Coal 

Min. and Proc, v. 20, Dec, 1983, pp,'49- 
51, 

7, Lewis, S., and L. R. Stace. Strata 
Substitution and Reinforcement Techniques 
in the United Kingdom. Paper in Pro- 
ceedings of the Seventh International 
Strata Conference (INIEX, Liege, Belgium, 
Sept. 1982). INIEX, Liege, Belgium, 
1982, 18 pp. 

8. Lawrence, R, G. , and R. King. 
Demonstration of Shield-Type Longwall 



Supports at York Canyon Mine of Kaiser 
Steel Corporation (U.S. Dep. Energy con- 
tract DE-ACO 1-7 4ET 12530, Kaiser Steel 
Corp.). Apr, 1980, 193 pp,; NTIS DOE/ET/ 
12530-Tl, 

9. Curth, E. Safety Aspects of Long- 
wall Mining in the Illinois Coal Basin. 
BuMines IC 8776, 1978, 37 pp. 

10, Moebs , N, , and E, Curth, Geologic 
and Ground Control Aspects of an Experi- 
mental Shortwall Operation in the Up- 
per Ohio Valley, BuMines RI 8112, 1976, 
30 pp, 

11, Barry, A, J,, 0. B, Nair, and 
J, S, Miller, Specifications for Se- 
lected Hydraulic-Powered Roof Supports 
With a Method To Estimate Support Re- 
quirements for Longwalls. BuMines IC 
8424, 1969, 15 pp, 

12, Wilson, A, H, Support Load Re- 
quirements on Longwall Faces, Min, Eng, 
(London), v, 134, June 1975, pp. 470- 
488. 

13, Peacock, A. Design of Shield Sup- 
ports for the U.S. Mining Industry. Pa- 
per in Proceedings of the First Annual 
Conference on Ground Control in Mining 
(WV Univ., Morgantown, WV, July 27-29, 
1981). WVUniv., Morgantown, WV, 1981, 
pp. 174-185. 



36 



14. Wade, L. V. Longwall Support Load 
Prediction From Geologic Information, 
Pres. at Soc. Min, Eng. AIME Fall Meet- 
ing, Denver, CO, Sept. 1-3, 1976. Soc. 
Min. Eng. AIME Preprint 76-1-308, 14 pp. 

15. Jacobi, 0. Praxis der Gebirgsbe- 
herrschung (Practice of Ground Control). 
Gluckauf, Essen, 1976, 494 pp. 

16. Gluckauf. Richtlinien Fur die 
Bauartzulassung von Schreitausbau (Guide- 
lines for the Approval of Powered Roof 
Support). V. 113, No. 29, 1977, p. 1015. 

17. Stokes, H. Modern Powered Sup- 
ports. Min. Eng. (London), v. 143, Aug. 

1983, pp. 51-58. 

18. Herwig, H. Die Wirkung des Geb- 
irgsdrucks auf den Hangendzustand im 
Streb (The Effect of Ground Stresses on 
Roof Conditions on a Longwall). Gluck- 
auf, V. 117, No. 21, 1981, pp. 1419- 
1423. 

19. Curth, E. Coal Mining Techniques 
in the Federal Republic of Germany — 1971. 
BuMines IC 8645, 1974, 52 pp. 

20. Everling, C. , and A. Meyer. Ein 
Gebirgsdruck-Rechenmodell als Planung- 
shilfe (A Mathematical Model for Ground 
Stress Distribution). Gluckauf Res. 
Rep,, V. 33, 1972, pp. 81-88. 

21. Janes, J. A Demonstration of 
Longwall Mining (contract J0333949, Old 
Ben Coal Co.). BuMines OFR 86(2)-85, 
Nov. 1983, 105 pp. 

22. Ratz, B. W, Proceedings of Long- 
wall Conference for Illinois Operations, 
West City, IL, 1979, 18 pp.; available 
upon request from E. A. Curth, BuMines, 
Pittsburgh, PA. 

23. . Wege zum Verbessern der 

Hangendbeherrschung im Gewinnungsf eld 
(Methods of Improving Roof Control on the 
Longwall Face), Gluckauf, v, 120, No, 3, 

1984, pp, 128-132. 

24. Brezovec, D. Martinka Avoids 
Headgate Dust, Coal Age, v, 85, Dec, 
1980, pp, 104-110, 

25. Buschmann, H, E, Elektrohydrau- 
lische Ausbausteuerungen bei der Bergbau 
AG, Niederrhein (Electrohydraulic Roof 
Support Controls at Niederrhein Mining 
Co,), Gluckauf, v, 120, No, 3, 1984, 
pp, 135-140, 



26, Irresberger, H, Schildausbau ein- 
facher, leichter? (Shield Support Sim- 
pler, Lighter?) Gluckauf, v, 118, No. 
18, 1982, pp. 927-933. 

27. Hahn, L. Stand und Kunftige Ent- 
wicklung des Strebausbaus (Status and Fu- 
ture Development of Longwall Roof Sup- 
port) . Gluckauf, V. 120, No. 3, 1984, 
pp. 157-162. 

28. Ratz, B. W. Die Weiterentwicklung 
des Schildausbaus in den 80er Jahren (Re- 
cent Development of Shield Support in 
the Eighties). Gluckauf, v. 119, No, 19, 

1983, pp, 925-929, 

29, Richardson, F, J, Application of 
CADD in the Mining Industry, Min, Eng, 
(London), v, 143, Dec. 1983, pp. 303-308. 

30, Allen, A, D, Modern Roof Support 
and the Effect of the Mining Department 
Instruction on the Use of Powered Roof 
Supports on Longwall Faces, Min, Tech- 
nol,, V, 65, Aug, 1983, pp, 309-312, 

31, Herms , W, Baumusterprufungen des 
Grubenausbaus im Zulassungsverfahren 
(Prototype Testing of Roof Support for 
Approval), Gluckauf, v. 120, No. 2, 

1984, pp. 84-90. 

32. Barczak, T. , and C. Goode. Con- 
siderations in the Design of Longwall 
Mining Systems. Ch. in State-of-the-Art 
of Ground Control and Mine Subsidence, 
ed. by Y, Chugh and M, Karmis, Soc, Min, 
Eng, AIME, 1982, pp, 39-50, 

33, Carson, R, , P, Yavorsky, T. Barc- 
zak, and Fuad Maayeh, State-of-the-Art 
Testing of Powered Roof Support, Paper 
in Proceedings of the Second Conference 
on Ground Control in Mining (WV Univ, , 
Morgantown, WV, July 19-21, 1982), WV 
Univ,, Morgantown, WV, 1982, pp, 64-69. 

34. Barczak, T. , and R. Carson. Tech- 
nique To Measure Resultant Load Vector on 
Shield Supports. Ch. in Rock Mechanics 
in Productivity and Protection, ed. by 
C. Dowding and M. Singh (Proc. 25th Symp. 
on Rock Mechanics, Northwestern Univ., 
Evanston, IL, June 25-27, 1984). Soc. 
Min. Eng. AIME, 1984, pp. 667-679. 

35. Fussel, W. , and F, Portge, Beh- 
errschung des Ausgasung durch Wetter- 
technische Zuschnitt des Abbaubetriebes 
(Methane Control by Planning the Face 



37 



Ventilation), Gluckauf , v. 112, No. 20, 
1976, pp. 1172-1174. 

36. Cavinder, M. Longwall Results in 
the Illinois Coal Basin. Min, Congr. J, , 
V. 68, Mar. 1982, pp. 37-40. 

37. Reynolds, J. First North American 
Longwall in Pitching Seams Proven Feasi- 
ble. Min. Eng. (Littleton, CO), v. 35, 
Dec. 1983, pp. 1615-1618. 

38. Wisecarver, D. , and J. Greenlee. 
Steep Seam Longwall. Ch, in Longwall- 
Shortwall Mining, State-of-the-Art , ed. 
by R. V. Ramani. Soc. Min. Eng. AIME, 

1981, pp. 211-215. 

39. Scheidat, L. Erfahrung rait Schil- 
dausbau in der geneigten Lagerung auf 
Erin (Experience With Shield Supports in 
Steeply Pitching Strata at Erin Mine) . 
Gluckauf, V. 120, No. 3, 1984, pp. 147- 
149. 

40. Adam, R. , and W. Douglas. Assess- 
ment of the Single Pass Thick Seam Long- 
wall Mining Method (U.S. Dep. Energy con- 
tract DE-AC01-79ET14246, Ketron, Inc.). 

1982, 192 pp.; NTIS DE 8300 1401. 

41. Goode, C, J. Jaspal, and T. Barc- 
zak. Support Selection for the Multilift 
Mining Method. Paper in Proceedings of 
the First Annual Conference on Ground 
Control in Mining (WV Univ. , Morgantown, 
WV, July 27-29, 1981). WV Univ., Morgan- 
town, WV, 1981, pp. 186-200. 

42. Drake, D. , and A. McCarthy. Re- 
view of Ten Feet Extraction at Daw Mill 



Colliery. Inst. Min. Eng,, Mar. 1978, 
42 pp.; available upon request from E. A. 
Curth, BuMines, Pittsburgh, PA. 

43. Bourquin, B. J., and J. S. Jaspal. 
Mid-Continent Has Early Success With the 
Longest Longwall Face Ever Operated in 
the U.S. Min. Eng. (Littleton, CO), v. 
36, Jan. 1984, pp. 48-52. 

44. Benech, M. Ein Hochleistungsstreb 
mit Abziehen der Hangendkohle in der Re- 
viergesellschaf t Blanzy (A Highly Produc- 
tive Longwall Face Extracting Coal by the 
Sublevel Method in the Blanzy Mining Dis- 
trict) . Gluckauf, V. 118, No. 13, 1982, 
pp. 646-649. 

45. U.S. Bureau of Mines. The Reserve 
Base of Bituminous Coal and Anthracite 
for Underground Mining in the Eastern 
United States. BuMines IC 8655, 1974, 
428 pp. 

46. Bergmann, M. , and H. Kundel. Die 
Tatigkeit des Arbeitskreises "Geringmach- 
tige Floze" in den Jahren 1978 bis 1982 
(Activity of the Task Force on Thin Seams 
During 1978-82). Gluckauf, v. 119, No. 
6, 1983, pp. 287-291. 

47. Curth, E. Longwall Mining of Thin 
Seams. Paper in Proceedings of the First 
Annual Conference of Ground Control in 
Mining (WV Univ., Morgantown, WV , July 
27-29, 1981). WV Univ., Morgantown, WV, 
1981, pp. 239-259. 



38 



APPENDIX A. — NATIONAL COAL BOARD MINING DEPARTMENT INSTRUCTION 
PI/ 1982/ 6: THE USE OF POWERED SUPPORTS ON LONQJALL FACES 



1. To ensure the effectiveness of sup- 
ports on longwall faces, this instruction 
and the associated Notes of Guidance lay 
down criteria to be taken into consid- 
eration when the coal face is being 
designed, 

2. Where powered supports approved for 
the purpose of Regulation 16(2) of the 
Coal and Other Mines (Support) Regula- 
tions 1966 are intended for use on long- 
wall faces planned to commence operation 
after 1 January 1983 systematic support 
systems shall be designed and provided in 
accordance with the standards laid down 
in this instruction, 

3. For the purpose of this instruction 
the face working shall comprise four 
zones of operation, namely: 

(a) the face line zone. 

(b) the buttress zone. 

(c) the pack zone. 

(d) the roadhead zone. 

4. Every face design shall separately 
specify for each of the zones the de- 
signed setting resistance and the de- 
signed yield resistance to be offered by 
the powered support system which shall 
not be less than the values given in the 
following table: 



Zone 



Resistance, mt/m^ 



Face 

Buttress. 

Pack 

Roadhead. 




15.0 H 

15.0 H 

10.0 H 

10.0 H 



H = designed extracted height, m. 
^Or 7.5 mt/m2 , whichever is greater. 
^Or 15.0 mt/m , whichever is greater. 

5. The designed distance between the 
centres of adjacent supports shall be 
stated and shall not normally exceed 1.5 
m. Provided, however, that where it is 
necessary to set props and bars between 
adjacent powered supports this dis- 
tance may be increased to accommodate 
them. 



6. In the roadhead zone where the pow- 
ered supports have been designed for that 
zone the distance between adjacent pow- 
ered supports shall not exceed that laid 
down by the manufacturer, 

7. Every face design shall specify the 
hydraulic supply system to the powered 
supports to achieve the designed setting 
resistance, 

8. Where the designed extracted height 
is less than 2,5 m the designed distance 
between the tip of the powered support 
roof beam and the coal face before normal 
cutting shall not exceed 0,4 m. Excep- 
tionally, and for the time being, where 
compliance with this requirement is im- 
practicable, the designed distance be- 
tween the tip of the powered support roof 
beam and the coal face may be increased 
by a distance not exceeding 0,1 m. 

9. Where the designed extracted height 
is 2,5 m or greater, the designed dis- 
tance between the tip of the powered sup- 
port roof beam and the coal face before 
normal cutting shall not exceed 0,5 m. 

10. Powered forepoles shall be provided 
on all powered supports, unless they are 
of an immediate forward support design, 
which are intended for use: 

(a) where the designed extracted 
height is 2.5 m or greater; or 

(b) where the designed depth of web 
exceeds 0.8 m. 

The forepoles shall be extended systemat- 
ically to provide support behind the coal 
getting machine so that the extended tip 
of each is not more than 0.1 m behind the 
centre of the roof. 

11. On all powered supports intended for 
use where the designed clearance between 
the top race of the armoured face con- 
veyor and the roof at the place exceeds 
2.3 m powered face sprags shall be pro- 
vided, and the Manager's Support Rules 
shall specify the system to be adopted 
for their use. 



39 



12. Where an immediate forward support 
system is used, the supports shall be ad- 
vanced as close as practicable behind the 
coal getting machine and normally this 
distance shall not exceed 10.0 m. 

13. The Manager's Support Rules shall 
include provision for the systematic sup- 
port of that part of every face where 
maintenance on coal getting machines or 
other work is required to be carried out 
in advance of the powered supports. 



14. Relaxation from this instruction or 
any part thereof may be granted only by 
the Director-General of Mining. 

15. Those to whom this instruction is 
distributed are reminded that it is their 
responsibility to bring its provisions to 
the notice of any members of their staff, 
not included in the distribution list, 
who are concerned with complying with 
this instruction and Notes of Guidance or 
taking action on it. 



40 

APPENDIX B. — SPECIFICATIONS FOR THYSSEN RHS 12/30 SHIELD 

Roof load characteristics: 

Leg capacity (LC) kN.. 4,780 

Vertical roof load at yield (VRL) , kN; 

Minimum at 1.2-m height 3,647 

Maximum at 2.6-^ height 4,697 

Roof load efficiency (VRL/LC) , pet: 

Minimum 76 

Maximum 98 

Mean load density at yield, 1 web back before cut: 

Web cm. . 76 

Minimum roof area at 1.2-m height , m^,, 5.14 

Mean load density .kPa. . 709 

Maximum roof area at 2.6-m height m^,, 5.20 

Mean load density kPa. . 903 

Base (divided type) : 

Bearing length m. . 2.30 

Overall length m. . 2.40 

Width of each skid m. . 0.55 

Effective bearing area m^.. 2.31 

Average floor pressure , N/cm^, . 183 

Maximum floor pressure N/cm^. . 310.5 

Gob shield (1 piece): 

Length m. . 2.29 

Width m. . 1.43 

Hinge arrangement Lemniscate 

Canopy: 

Length without flipper extension m, , 2.40 

Length of flipper m. . 0.76 

Total length m. . 3. 16 

Width m. . 1.43 

Ratio of fore part to rear part, related to resultant location 2.2 

Span of canopy tip to coal face m,, 0.30 

Flipper: 

Angle of uplift deg. . +20 

Tip load when horizontal kN. . 71 

Cylinders 2 

Force of 2 cylinders at 37 MPa kN.. 351 

Canopy cylinder: 

Number 1 

Set: Piston at 37 MPa kN. . 651 

Retract: Rod at 37 MPa kN.. 363 

Yield: Piston at 45 MPa kN. . 792 

Advancing cylinder (reverse linkage) : 
Force, kN: 

Piston (shield pull) 351 

Rod (conveyor push) 163 

Effective stroke m. . 0.76 

Leg (double-telescoping, double-action) : 
Length, m: 

Closed 1.14 

Extended 2.76 



41 



Leg — Cont inued 

Piston area, cm^ : 

1st stage 531 

2d stage 531 

Operating pressure in both stages: 

Set MPa. . 37 

Yield MPa. . 45 

Set-yield ratio pet. . 82 

Force, kN: 

Set 1,966 

Yield 2, 390 

Surface finish ym. . 4 

Plating material Nickel 

Plating thickness ytn. . 30-50 

Steel quality of structural components (canopy, gob shield, base, 
lemniscate) , MPa: 

Links 345 

Legs 490 

Pins alloy : 50 VCrVi^ 980 

Structure Box welded 

Side sealing (1 side, plates on canopy and gob shield): 

Springs 4 

Cylinders 4 

Force of 1 cylinder: push kN.. 120 

Travelway width, m: 

At base 0.55 

1.14 m above base 0.60 

Hydraulics: 
Diameter, mm: 

Pressure line 25 

Re turn 1 ine 31 

Fluid (oil-in-water) pet, . 2-5 

Operating pressure MPa. . 37 

Adjacent control 
Full flow valve 
Steck-o coupling 
General information: 

Weight mt . . 13 

Range: 

Closed , m. . 1.2 

Extended , m. . 3 

Overall length, flipper retracted m. . 4.65 

Overall width m. . 1.43 

Width with side plates extended m.. 1.68 

Automatic contact advance under 9.6-kPa pressure against the roof, water sprays, 
pressure indicator on each leg, mounting plates for light fixtures. 



42 



APPENDIC C. — MANUFACTURERS 



Home Office 

Babcock Roof Supports Ltd. 

Aidan House, Tynegate Precinct 

Sunderland Rd, , Gateshead 

Tyne & Wear NE8 3HY, United Kingdom 



U.S. Representation 



Huwood-Irwin Co, 
P.O. Box 409 
Irwin, PA 15642 
Tel. (412) 863-5000 



Bochumer Eisenhutte-Heintzmann GmbH 

POB 101029 

D-4630 Bochum 1 

Federal Republic of Germany 

Dowty Mining Equipment Ltd, 

Ashchurch 

Gloucestershire GL20 8JR, United Kingdom 



Heintzmann Corp, 
P.O. Box 1027 
Lebanon, VA 24266 
Tel. (703) 889-5533 

Dowty Corp. 

177 Thorn Hill Rd. 

Thorn Hill Industrial Park 

Warrendale, PA 15086 

Tel. (412) 776-3693 



Gullick Dobs on Ltd. 

P.O. Box 12 

Ince , Wigan 

Greater Manchester WNI 3DD 

United Kingdom 

Hemscheidt Maschinenf abrik 
Bornberg 97-103 
D-5600 Wuppertal 1 
Federal Republic of Germany 



Joy Manufacturing Co. 

Oliver Bldg. 

Pittsburgh, PA 15222, U.S.A. 

Klockner-Becorit 

PF 209 

D-4350 Recklinghausen 

Federal Republic of Germany 



Gfullick Dobson, Inc. 
603 Parkway View Dr. 
Pittsburgh, PA 15205 
Tel. (412) 787-5852 



Heimscheidt America Corp, 
252 Parkwest Dr. 
P.O. Box 500 
Pittsburgh, PA 15230 
Tel. (412) 787-7130 



U.S. producer 



Kloeckner-Becorit 
North America, Inc. 
790 Manor Oak Two 
1910 Cochran Rd. 
Pittsburgh, PA 15220 
Tel. (412) 344-8200 



Machinoexport 

35, Mosf ilmovskaya ul. 

117330 Moscow, U.S.S.R. 



None 



Marrel Mines - Bennes Marrel S.A. 

ZI St-Etienne Boutheon 

BP 56-42160 Andrezieux Boutheon, France 



None 



43 



Home Office 

Material de Fond et d' Industrie 
rue de Chsmps de Mars 
57202, Sarreguemlnes , France 

Mitsui Mllke Machine Co. Ltd. 
2, Asahl-Machl, Omuta-Clty 
Fukuoka, Japan 



Thyssen Bergbautechnlk 

PF 281144 

D-4100 2 Dulsburg 

Federal Republic of Germany 



Westfalla Lunen 

D-4670 Lunen 

Federal Republic of Germany 



U.S. Representation 



None 



Long Alrdox Co. 

Robinson Plaza III, Suite 320 

Rte. 60, Robinson Township 

Pittsburgh, PA 15205 

Tel. (412) 787-8292 

Thyssen Mining Equipment 
Dlv. of Thyssen, Inc. 
Stanley Bldg. , Suite 302 
Marlon, IL 62959 
Tel. (618) 997-5328 

Mining Progress, Inc. 
300 Boulevard Tower 
Charleston, WV 25301 
Tel, (304) 343-5593 



U.S. Department of the Interior 
Bureau of Mines— Prod, and Distr. 
Cochrans Mill Road 
P.O. Box 18070 
Pittsburgh. Pa. 15236 



AN EQUAL OPPORTUNITY EMPLOYER 



OFFICIAL BUSINESS 
PENALTY FOR PRIVATE USE. S300 

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HECKMAN 

BINDERY INC. 

€^JUN 86 
W N. MANCHESTER, 
^ INDIANA 46962 










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